perf(gateway): trim event/command hot-path allocations (GWC-06/07/15, IPC-05)

Behavior-preserving allocation cuts on the per-event/per-command path:
- GWC-06: StreamEvents send-timing uses Stopwatch.GetTimestamp() +
  GetElapsedTime() instead of a per-event Stopwatch allocation (same measured
  span).
- GWC-07/IPC-05 (event): MapEvent transfers ownership of the inner MxEvent
  instead of .Clone()-ing it. Safe: WorkerEvent is parsed fresh per pipe frame
  with the distributor pump as its single consumer (GWC-01), MapEvent runs once
  before fan-out, and every downstream consumer (subscribers, replay ring)
  only READS the event (WorkerSequence is stamped upstream; verified no
  post-mapping mutation). Comment documents the invariant + restore-clone
  caveat if a second consumer is added.
- IPC-05 (command): CreateCommandEnvelope no longer re-clones; MapCommand
  already isolated the graph from the caller-owned gRPC message.
- GWC-15: grpc_stream_queue.depth converts from a per-event push counter to an
  ObservableGauge summing registered channel sources at scrape time only
  (name/semantics unchanged); removes all per-event .Count/lock work.

Kept every load-bearing isolation clone (MapCommand, Invoke, bulk filters,
MapCommandReply). Server build clean (0 warnings); EventStream/Metrics/
Distributor/Mapper tests 62/62 (incl. formerly-flaky queue-depth tests, now
green under the lazy gauge, + 2 new MapEvent ownership tests). Docs: Metrics.md,
Grpc.md.

Claude-Session: https://claude.ai/code/session_01DMXXvNuPekkkrTEyPNxEkW

docs/Grpc.md

@@ -215,6 +215,10 @@ public WorkerCommand MapCommand(MxCommandRequest request)
 }
 ```
 
+The command clone above is the only isolating copy on the command path. `WorkerClient.CreateCommandEnvelope` no longer re-clones the `WorkerCommand` into the outbound envelope: the mapper's clone already produced a fresh graph owned by the invoke pipeline, and the envelope is built and owned entirely inside `WorkerClient`, so a second copy would be redundant.
+
+`MapEvent` takes the opposite stance from `MapCommand`: it transfers ownership of the inner `MxEvent` rather than cloning it. The enclosing `WorkerEvent` is parsed fresh from a single pipe frame and the `SessionEventDistributor` pump is its single consumer, so nothing else aliases or mutates it. The pump then shares that one `MxEvent` across every subscriber and the replay ring, but that fan-out is read-only, so a single instance is safe. If a second consumer of the `WorkerEvent` is ever introduced, restore the clone in `MapEvent`.
+
 When the worker reply or event payload is missing, the mapper returns a synthetic public message with `ProtocolStatusCode.ProtocolViolation` (for replies) or a sentinel `MxEvent` with `MxEventFamily.Unspecified` (for events). The gateway never relays a partial frame to clients — anything missing is reported as a protocol violation against the worker, not a transport error against the client.
 
 The mapper also exposes static factory methods for every `ProtocolStatusCode` (`Ok`, `InvalidRequest`, `SessionNotFound`, `SessionNotReady`, `WorkerUnavailable`, `Timeout`, `Canceled`, `ProtocolViolation`) so that handlers and tests can produce status payloads without duplicating the enum-to-string mapping.