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/WindowsDeviceKeyStore.cs

@@ -9,9 +9,17 @@ namespace DodoSSH.Client.App.Platform;
 /// </summary>
 /// <remarks>
 /// <para>
-/// One place decides, so nothing above has to carry a platform guard. A machine with no TPM, or one that
-/// is not Windows, gets <see cref="UnavailableDeviceKeyStore"/> and therefore keeps asking for the
-/// passphrase — which is the honest answer rather than a degraded one.
+/// One place decides, so nothing above has to carry a platform guard. A machine with no secure hardware,
+/// or one that is neither Windows nor macOS, gets <see cref="UnavailableDeviceKeyStore"/> and therefore
+/// keeps asking for the passphrase — which is the honest answer rather than a degraded one.
+/// </para>
+/// <para>
+/// <b>Both real stores are asked whether they work rather than told that they do.</b> Each
+/// <c>IsSupported</c> probes by doing the thing — creating a throwaway key and deleting it — because on
+/// both platforms the provider is present and reports itself present on machines where creating a key
+/// fails: a Windows box with no usable TPM, a Mac with no Secure Enclave, and on macOS also every
+/// unsigned development build, since enclave keys need a signing identity. Inferring from the OS would
+/// mean each of those discovering the truth at the moment somebody tried to unlock.
 /// </para>
 /// <para>
 /// <b>"Desktop", because the choice belongs to a head rather than to the session layer.</b> This file used
@@ -29,9 +37,17 @@ public static class DesktopDeviceKeyStores
     {
         ArgumentNullException.ThrowIfNull(paths);
 
-        return OperatingSystem.IsWindows() && WindowsDeviceKeyStore.IsSupported()
-            ? new WindowsDeviceKeyStore(paths)
-            : new UnavailableDeviceKeyStore();
+        if (OperatingSystem.IsWindows() && WindowsDeviceKeyStore.IsSupported())
+        {
+            return new WindowsDeviceKeyStore(paths);
+        }
+
+        if (OperatingSystem.IsMacOS() && MacDeviceKeyStore.IsSupported())
+        {
+            return new MacDeviceKeyStore(paths);
+        }
+
+        return new UnavailableDeviceKeyStore();
     }
 }