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/shared/MoShiro.Measurement/Update/UpdateBridge.cs

@@ -96,34 +96,5 @@ public sealed class UpdateBridge
         }
     }
 
-    /// <summary>
-    /// 握手时自述的二进制：正在运行的可执行文件的哈希，加上宿主机更新器验过的、安装这个版本时用的 CI 签名清单（更新器写在只读的 <c>update-status/</c>）。
-    /// 容器节点以外（没有更新器）没有清单，只有哈希；读不到可执行文件时返回 null（不做绑定）。
-    /// </summary>
-    public BinaryAttestation? Attest()
-    {
-        if (NodeBinary.Sha256Hex is not { } hash)
-        {
-            return null;
-        }
-
-        string? manifest = ReadSmall("installed.manifest", MaxManifestBytes);
-        string? signature = ReadSmall("installed.sig", MaxSignatureBytes);
-        return manifest is not null && signature is not null ? new BinaryAttestation(hash, manifest, signature) : new BinaryAttestation(hash, null, null);
-    }
-
-    private string? ReadSmall(string name, int maxBytes)
-    {
-        try
-        {
-            string path = Path.Combine(Path.GetDirectoryName(_statusFile)!, name);
-            return File.Exists(path) && new FileInfo(path).Length <= maxBytes ? File.ReadAllText(path, Encoding.UTF8) : null;
-        }
-        catch (Exception ex) when (ex is IOException or UnauthorizedAccessException)
-        {
-            return null;
-        }
-    }
-
     private static bool Same(string path, byte[] content) => File.Exists(path) && File.ReadAllBytes(path).AsSpan().SequenceEqual(content);
 }