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/BackendFixture.cs

@@ -265,7 +265,8 @@ public class BackendFixture : IAsyncLifetime
                 options.RequireOwner = false; // 大部分测试不建立信任根；属主必选的测试自己打开（OwnerTests）
                 options.GeoLookupUrl = string.Empty; // 测试里不访问外部服务
                 options.PhaseDurationScale = 0.2; // 端到端测试里 connect、hold 这些固定 30 秒的阶段缩成 6 秒
-            });
+            },
+            configureServices: services => services.AddSingleton<MoShiro.Backend.Application.IReleaseKey>(TestReleaseKey.Instance)); // 登记清单要验发布签名：测试用自己的密钥签
         await App.StartAsync();
         string address = App.Services.GetRequiredService<IServer>().Features.Get<IServerAddressesFeature>()!.Addresses.First();
         BaseAddress = new Uri(address);