refactor: remove the runtime binary binding; every release registration path verifies the CI signature [CI]

The hash a node reports about itself cannot be trusted (a swapped binary can
copy the old hash), so the runtime binding only caught files that were replaced
by something that did not lie. Provenance stays where it can be verified: the
signed release manifest (digest) and the signed SHA256SUMS.

- Remove BinaryBindingService, the node_bindings table (schema v15 drops it),
  BinaryPolicy, the handshake hash/manifest fields and the admin binary column.
- Drop the per-platform "binaries" list from the release manifest, the hash
  scripts and the updater's installed.manifest copy.
- ReleaseService.PublishAsync now verifies the release signature, so the admin
  POST /releases route is no longer an unsigned path; tests sign with a test key.
- Docs: section 16 explains why the runtime check is gone.

backend/tests/MoShiro.Backend.Tests/ReleaseOfferTests.cs

@@ -74,6 +74,22 @@ public sealed class ReleaseOfferTests
         Assert.Equal(HttpStatusCode.Forbidden, response.StatusCode);
     }
 
+    [Fact]
+    public async Task OfferWithAValidSignature_IsAccepted_WithoutAnyToken_AndTheAdminEndpointChecksTheSignatureToo()
+    {
+        long seq = TimeProvider.System.GetUtcNow().ToUnixTimeSeconds() * 1000; // backend 的 release 通道：不和别的用例抢 agent、probe 的 seq
+        string manifest = $$"""{"schema":1,"component":"backend","channel":"release","image":"registry.example/moshiro/moshiro-backend","digest":"sha256:0000000000000000000000000000000000000000000000000000000000000000","tag":"dev-x","version":"0.1.1","seq":{{seq}}}""";
+        using HttpClient anonymous = _backend.NewClient();
+
+        using HttpResponseMessage offered = await anonymous.PostAsync("/api/release/offer", BackendFixture.Json(new JsonObject { ["manifest"] = manifest, ["signature"] = TestReleaseKey.Sign(manifest) }));
+        Assert.Equal(HttpStatusCode.Created, offered.StatusCode);
+
+        // 管理员通道不再是不验签的口子：签名不对一样拒绝
+        string next = manifest.Replace($"\"seq\":{seq}", $"\"seq\":{seq + 1}", StringComparison.Ordinal);
+        using HttpResponseMessage forged = await _backend.Admin.PostAsync("/api/admin/releases", BackendFixture.Json(new JsonObject { ["manifest"] = next, ["signature"] = OpenSslSignature }));
+        Assert.Equal(HttpStatusCode.Forbidden, forged.StatusCode);
+    }
+
     [Fact]
     public async Task TheAdminEndpoint_StillNeedsTheCapability()
     {