Restructure into src/tests and add build foundation (M0)

Moves the scaffold to src/DodoSSH.Api and establishes the repo conventions the rest
of the milestones build on.

Structure:
- src/{Contracts,Crypto,Domain,Infrastructure,Api}, tests/{Contracts,Crypto,Domain}.Tests
- DodoSSH.slnx rewritten with src/ and tests/ solution folders

Build:
- Directory.Build.props centralises TFM, nullable, deterministic builds and
  TreatWarningsAsErrors; Directory.Packages.props pins every version centrally
- packages.lock.json committed so CI restores in locked mode
- NuGet.config clears machine-level sources, which both fixes NU1507 under central
  package management and makes restore reproducible off this machine
- Microsoft.OpenApi pinned to 2.11.0: ASP.NET Core 10.0.10 resolves 2.0.0, which is
  covered by GHSA-v5pm-xwqc-g5wc (high, patched in 2.7.5)

Analyzers:
- AnalysisLevel is Recommended, not All. With warnings-as-errors, All turns opinionated
  naming rules into build breaks and trains people to blanket-suppress.
- BannedSymbols.txt bans DateTime.UtcNow (TimeProvider), Guid.NewGuid (CreateVersion7),
  sync-over-async, MD5/SHA1, PBKDF2 and SecureString
- CA1711/CA1724 disabled: both are .NET Framework CAS-era naming rules
- PublicApiAnalyzers on Contracts only, since that assembly is the client's real contract

API:
- weather-forecast template removed
- UseHttpsRedirection removed; TLS terminates at the reverse proxy and redirecting
  behind one causes loops
- /healthz/{live,ready,startup}. Liveness deliberately checks no dependencies so a
  transient database outage cannot restart the container and kill live SSH sessions.

Notes:
- No coverage collector yet. Microsoft.Testing.Extensions.CodeCoverage pulls an MTP 1.x
  MSBuild extension that throws TypeLoadException against the MTP 2.3.x xunit.v3 brings.
  Coverage gates are an M3 concern; revisit with an MTP 2.x-aligned version then.

Verified: dotnet build (0 warnings), 17 tests pass, format check clean, API serves
health and OpenAPI endpoints.

src/DodoSSH.Crypto/CryptoSpec.cs

@@ -0,0 +1,47 @@
+namespace DodoSSH.Crypto;
+
+/// <summary>
+/// Constants of the DodoSSH cryptographic specification.
+/// </summary>
+/// <remarks>
+/// docs/crypto.md is the normative specification; this type must agree with it exactly.
+/// The implementation of the envelope, AAD derivation and key wrapping lands in M1, once
+/// the specification and its test vectors are frozen. Nothing else may be built on top of
+/// an unfrozen AAD: only clients can re-encrypt, so a change after users hold data cannot
+/// be migrated server-side.
+/// </remarks>
+public static class CryptoSpec
+{
+    /// <summary>Magic prefix identifying a DSH1 envelope.</summary>
+    public const string EnvelopeMagic = "DSH1";
+
+    /// <summary>Version of the AAD derivation rule that payloads are bound to.</summary>
+    /// <remarks>
+    /// Stored per row as <c>payload_aad_version</c> so a future change can be applied
+    /// lazily, re-encrypting on next write rather than in a migration.
+    /// </remarks>
+    public const short CurrentAadVersion = 1;
+
+    /// <summary>Domain-separation prefix for every AAD computation.</summary>
+    public const string AadDomainPrefix = "dsh1\n";
+
+    /// <summary>Identifiers for the algorithms an envelope may declare.</summary>
+    public enum AlgorithmId : byte
+    {
+        /// <summary>Reserved; never written.</summary>
+        Unspecified = 0,
+
+        /// <summary>Symmetric content encryption under a known key.</summary>
+        XChaCha20Poly1305 = 1,
+
+        /// <summary>Symmetric fallback where XChaCha20 is unavailable.</summary>
+        Aes256Gcm = 2,
+
+        /// <summary>Anonymous-sender seal to an X25519 public key.</summary>
+        SealToX25519 = 3,
+
+        // 4 is reserved for a hybrid X25519 + ML-KEM-768 seal. Store-now-decrypt-later is
+        // a genuine threat for long-lived SSH keys, so the identifier is claimed now even
+        // though the construction ships later.
+    }
+}