feat(consumergate): runtime stop/start of queue controllers

[sbalabanov]

Implements the consumer-gate RFC (#374) and showcases a Request Cancellation e2e test using it.

Summary

• Adds platform/extension/consumergate with the Gate, Entry, Admin, Factory, and configuration contracts, plus generated mocks.
• Adds file-backed and no-op implementations. The file backend stores gate state and currently parked deliveries in a shared directory using atomic temp-file-plus-rename writes.
• Clears every queue delivery through its consumer-group/partition gate before controller processing. Blocked deliveries remain in flight through visibility extensions without consuming retry attempts or breaking partition ordering.
• Fails open when gate reads, monitoring, or visibility extensions fail. Consumer shutdown instead leaves a blocked delivery unacked for normal redelivery.
• Keeps parked records only while a delivery is actively blocked.
• Wires the shared file gate into gateway, orchestrator, and runway. Stovepipe uses the no-op implementation.
• Runs application containers under the host test UID/GID with rootful Docker, while using 0:0 under rootless Docker so bind-mounted gate files remain readable by the host test process.

Deterministic cancellation E2E coverage

TestCancel_CaughtPreBatch_NeverLands closes the runway merge-conflict-check gate for the test queue before landing a request. The test observes the exact parked delivery, cancels the request to its terminal state, opens the gate, waits for the parked record to disappear, and uses a sentinel request to prove the stale message was consumed without reviving or landing the cancelled request.

Testing

• make test — passed (76/76 Bazel test targets).
• //test/integration/... — passed (8/8 integration targets).
• //test/e2e/... — passed (2/2 E2E targets).
• git diff --check — passed.

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