Give the desktop a macOS head, signed from the first release

The same application, the same Velopack and the same two-phase person-run
release as Windows, with four things forced to differ. Signing is a
precondition rather than an improvement: Gatekeeper refuses an
un-notarized download outright instead of warning about it, so there was
never the "unsigned for now" that ADR 0013 decision 8 argues for on
Windows, and release-macos.sh refuses to start without the identities.

The packaging split is narrower than it first looked, and the old claim
at the foot of ci.yml is why it was worth checking rather than assuming.
vpk cross-compiles when told to: 'vpk [osx] bundle' builds a real .app on
any platform, and CI now publishes osx-arm64 and bundles it on every main
and tag build, which is what catches a restore graph with no macOS native
asset. There is no '[osx] pack' off a Mac, and that part is correct — pack
drives codesign, notarytool and stapler, which exist nowhere else.

The dylib signing loop in the script looks redundant beside vpk's own
pass and is not. vpk signs with 'codesign --deep', which is the shape
Apple documents as wrong for nested code, and platform-flags has recorded
a notarization rejection that names no file since before any of this
existed. Signing each native binary inside-out first leaves that pass
nothing to get wrong.

MacDeviceKeyStore reaches ADR 0007's conclusion through different
hardware: a P-256 key in the Secure Enclave under an access control
requiring user presence, so the platform enforces the gate rather than
this process — which is the whole point of that ADR's amendment. The
enclave holds no other kind of key, hence ECIES where Windows uses
RSA-OAEP, and the shape that falls out is better than the Windows one:
sealing needs only the public half and is silent, so only unlock prompts.
IsSupported probes rather than infers, because three ordinary Macs answer
no — an Intel machine without a T2, one with no login password, and every
unsigned development build, since enclave keys need a signing identity.

Two decisions worth stating because they are reversible. arm64 only: a
second channel is small work and nobody here has an Intel Mac to walk
Phase 18 on, and an x64 package would be the only artefact in this
repository reaching users unverified. And the pack id stays
DodoSSH.Desktop even though vpk names the bundle after it, so
/Applications holds DodoSSH.Desktop.app: decision 2's reasoning binds
harder here, because a pack id of DodoSSH would put Velopack's install
root on top of ClientPaths.DataDirectory and let an uninstall take the
user's un-synced outbox with it. CFBundleDisplayName puts the product
name back in front of a person.

Measured rather than assumed, since none of it is obvious: the publish
and the bundle were both run, LSMinimumSystemVersion is 12.0 because that
is the minos in the apphost's own LC_BUILD_VERSION, and vpk copies a
custom Info.plist verbatim with no substitution at all — which is why the
plist is a template the script renders and not a committed file.

What is not done is the half that needs the hardware. There is no macOS
runner, so nothing past "it bundles" has ever run. Phase 18 is the whole
of the verification, and the two checks most likely to fail are the
terminal against WKWebView and the enclave interop, neither of which has
executed once.

src/DodoSSH.Client.App/Platform/MacSecurity.cs

@@ -0,0 +1,254 @@
+using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
+
+namespace DodoSSH.Client.App.Platform;
+
+/// <summary>
+/// The pieces of CoreFoundation and Security.framework <see cref="MacDeviceKeyStore"/> needs.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Separated from the store itself because it is a different kind of code with a different kind of
+/// review: nothing here makes a decision, and everything here is a translation of a C declaration that
+/// is either right or wrong. Mixing the two would mean the security argument in
+/// <see cref="MacDeviceKeyStore"/> had to be read past two hundred lines of marshalling to find.
+/// </para>
+/// <para>
+/// <b>Every Create or Copy returns an object this process owns.</b> That is CoreFoundation's Create
+/// Rule, and it is the thing here that goes wrong silently: the enclave key handle is small, so a leak
+/// shows up as nothing at all until a long-running process has done a few thousand unlocks.
+/// <see cref="CoreFoundationScope"/> exists so ownership is tracked by construction rather than by
+/// remembering, and every function below that returns a handle says whether it is owned.
+/// </para>
+/// <para>
+/// <b>The integer widths are the part worth checking against the headers rather than skimming.</b>
+/// <c>CFIndex</c>, <c>CFOptionFlags</c> and <c>CFNumberType</c> are all pointer-width on a 64-bit Mac,
+/// not 32-bit, and getting one wrong does not fail cleanly — it shifts every argument after it, so the
+/// call receives plausible rubbish and returns a parameter error that names nothing.
+/// </para>
+/// </remarks>
+[SupportedOSPlatform("macos")]
+internal static partial class MacSecurity
+{
+    internal const string SecurityFramework =
+        "/System/Library/Frameworks/Security.framework/Security";
+
+    internal const string CoreFoundation =
+        "/System/Library/Frameworks/CoreFoundation.framework/CoreFoundation";
+
+    /// <summary>
+    /// The access control flags <c>SecAccessControlCreateWithFlags</c> takes.
+    /// </summary>
+    /// <remarks>
+    /// <c>ulong</c> because the parameter is a <c>CFOptionFlags</c>, which is an <c>unsigned long</c>.
+    /// Only the two flags that are used are listed; the full set is large, and copying it in would
+    /// invite somebody to reach for one without reading what it does to the prompt — <c>Biometry</c>
+    /// alone, for instance, leaves a Mac with no Touch ID unable to unlock at all rather than falling
+    /// back to the login password.
+    /// </remarks>
+    [Flags]
+    internal enum AccessControlFlags : ulong
+    {
+        /// <summary>
+        /// Touch ID if the machine has it, the login password if not.
+        /// </summary>
+        /// <remarks>
+        /// The forgiving one, deliberately. <c>BiometryCurrentSet</c> would additionally invalidate the
+        /// key whenever a fingerprint is added or removed, which sounds stricter and here buys nothing:
+        /// this key wraps a device key whose loss already means "ask for the passphrase", so the only
+        /// effect would be users being sent back to their passphrase by an unrelated Settings change
+        /// they would never connect to it.
+        /// </remarks>
+        UserPresence = 1ul << 0,
+
+        /// <summary>Required for any key that lives in the Secure Enclave.</summary>
+        PrivateKeyUsage = 1ul << 30,
+    }
+
+    /// <summary>The CFNumberType code for a 32-bit int, from CFNumber.h.</summary>
+    internal const long CFNumberIntType = 9;
+
+    /// <summary>errSecSuccess.</summary>
+    internal const int Success = 0;
+
+    /// <summary>errSecItemNotFound, which is an answer rather than a failure.</summary>
+    internal const int ItemNotFound = -25300;
+
+    // ---- CoreFoundation ------------------------------------------------------------------------------
+
+    /// <summary>Releases an owned handle.</summary>
+    [LibraryImport(CoreFoundation)]
+    internal static partial void CFRelease(IntPtr handle);
+
+    /// <summary>Copies bytes into a new CFData. Owned.</summary>
+    [LibraryImport(CoreFoundation)]
+    internal static partial IntPtr CFDataCreate(IntPtr allocator, IntPtr bytes, nint length);
+
+    [LibraryImport(CoreFoundation)]
+    internal static partial IntPtr CFDataGetBytePtr(IntPtr data);
+
+    [LibraryImport(CoreFoundation)]
+    internal static partial nint CFDataGetLength(IntPtr data);
+
+    /// <summary>Boxes a value as a CFNumber. Owned.</summary>
+    [LibraryImport(CoreFoundation)]
+    internal static partial IntPtr CFNumberCreate(IntPtr allocator, nint theType, IntPtr valuePtr);
+
+    /// <summary>Builds an immutable dictionary. Owned.</summary>
+    /// <remarks>
+    /// <para>
+    /// The key and value arrays are passed as raw pointers to memory the caller pins, rather than as
+    /// managed arrays. Source-generated interop wants an explicit element count for a marshalled array,
+    /// and supplying one here would mean stating the length twice — once for the marshaller and once as
+    /// <paramref name="numValues"/> — which is exactly the pair that drifts.
+    /// </para>
+    /// <para>
+    /// The two callback tables are what make the dictionary retain its keys and values, which is why
+    /// they are passed rather than left null: with null callbacks the dictionary stores raw pointers and
+    /// keeps nothing alive, and the resulting use-after-free is intermittent by nature.
+    /// </para>
+    /// </remarks>
+    [LibraryImport(CoreFoundation)]
+    internal static partial IntPtr CFDictionaryCreate(
+        IntPtr allocator,
+        IntPtr keys,
+        IntPtr values,
+        nint numValues,
+        IntPtr keyCallBacks,
+        IntPtr valueCallBacks);
+
+    // ---- Security.framework --------------------------------------------------------------------------
+
+    /// <summary>Builds the access policy a Secure Enclave key is created under. Owned.</summary>
+    [LibraryImport(SecurityFramework)]
+    internal static partial IntPtr SecAccessControlCreateWithFlags(
+        IntPtr allocator,
+        IntPtr protection,
+        AccessControlFlags flags,
+        out IntPtr error);
+
+    /// <summary>Creates a key pair from an attribute dictionary. Owned.</summary>
+    [LibraryImport(SecurityFramework)]
+    internal static partial IntPtr SecKeyCreateRandomKey(IntPtr parameters, out IntPtr error);
+
+    /// <summary>The public half of a key. Owned.</summary>
+    /// <remarks>
+    /// Available even for an enclave key, and that asymmetry is the whole reason this design works: the
+    /// public half is an ordinary key this process can hold and use, while the private half is a handle
+    /// to something inside the enclave that never becomes bytes. So sealing is silent and unsealing is
+    /// the thing the user is asked about.
+    /// </remarks>
+    [LibraryImport(SecurityFramework)]
+    internal static partial IntPtr SecKeyCopyPublicKey(IntPtr key);
+
+    /// <summary>Encrypts with a public key. Owned.</summary>
+    [LibraryImport(SecurityFramework)]
+    internal static partial IntPtr SecKeyCreateEncryptedData(
+        IntPtr key,
+        IntPtr algorithm,
+        IntPtr plaintext,
+        out IntPtr error);
+
+    /// <summary>Decrypts with a private key, prompting for whatever guards it. Owned.</summary>
+    [LibraryImport(SecurityFramework)]
+    internal static partial IntPtr SecKeyCreateDecryptedData(
+        IntPtr key,
+        IntPtr algorithm,
+        IntPtr ciphertext,
+        out IntPtr error);
+
+    /// <summary>Finds a keychain item. The out handle is owned when the result is <see cref="Success"/>.</summary>
+    [LibraryImport(SecurityFramework)]
+    internal static partial int SecItemCopyMatching(IntPtr query, out IntPtr result);
+
+    /// <summary>Deletes every keychain item matching the query.</summary>
+    [LibraryImport(SecurityFramework)]
+    internal static partial int SecItemDelete(IntPtr query);
+
+    // ---- The framework constants ---------------------------------------------------------------------
+
+    /// <summary>
+    /// Reads one of a framework's global CFString constants, or zero if it is not exported.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// The keys these dictionaries take are not strings this code may spell for itself. They are
+    /// pointer-comparable constants exported by the framework, and a CFString built here with the same
+    /// characters is a different object — the lookups would miss and the call would fail with a
+    /// parameter error naming nothing.
+    /// </para>
+    /// <para>
+    /// <b>Dereferenced once, because the exported symbol is the variable rather than its value.</b>
+    /// <c>TryGetExport</c> answers the address of the global; the CFStringRef is what that address
+    /// holds. Missing the indirection produces a pointer that is stable, plausible and wrong, which is
+    /// the worst of the three available outcomes.
+    /// </para>
+    /// <para>
+    /// Zero on a missing symbol rather than an exception, because the caller's answer to every failure
+    /// is the same one — report the store unavailable and let unlock ask for the passphrase — and a
+    /// constant that has been renamed by a future macOS should reach that answer rather than a crash.
+    /// </para>
+    /// </remarks>
+    internal static IntPtr Constant(IntPtr library, string symbol) =>
+        NativeLibrary.TryGetExport(library, symbol, out var address)
+            ? Marshal.ReadIntPtr(address)
+            : IntPtr.Zero;
+}
+
+/// <summary>
+/// Releases every CoreFoundation handle put into it, in reverse order, exactly once.
+/// </summary>
+/// <remarks>
+/// <para>
+/// The alternative is a try/finally per handle, and the operations here need six or seven at a time — a
+/// dictionary holding a nested dictionary holding an access control object holding a CFData tag. Finallys
+/// nested that deep stop being read, and a handle released twice is a crash rather than a leak.
+/// </para>
+/// <para>
+/// <see cref="Keep"/> returns what it was given, so a handle can be tracked in the same expression that
+/// produces it and the call sites read as ordinary code.
+/// </para>
+/// </remarks>
+[SupportedOSPlatform("macos")]
+internal sealed class CoreFoundationScope : IDisposable
+{
+    private readonly List<IntPtr> owned = [];
+
+    private bool disposed;
+
+    /// <summary>Takes ownership of a handle and hands it straight back.</summary>
+    /// <remarks>
+    /// Zero is ignored rather than rejected. Every CoreFoundation call here answers zero on failure, so
+    /// accepting it lets a caller track the result in the expression that produces it and check it on
+    /// the next line, instead of writing the check twice.
+    /// </remarks>
+    internal IntPtr Keep(IntPtr handle)
+    {
+        if (handle != IntPtr.Zero)
+        {
+            owned.Add(handle);
+        }
+
+        return handle;
+    }
+
+    public void Dispose()
+    {
+        if (disposed)
+        {
+            return;
+        }
+
+        disposed = true;
+
+        // Reverse order, so a container is released before the things it retains. CoreFoundation does not
+        // require it — retain counts make the order irrelevant — but it keeps the lifetimes readable in a
+        // debugger, where a released container that still lists its contents is a confusing thing to meet.
+        for (var i = owned.Count - 1; i >= 0; i--)
+        {
+            MacSecurity.CFRelease(owned[i]);
+        }
+
+        owned.Clear();
+    }
+}