feat(queue): replace the best-effort transient-lock PR mutex with the documented SubmissionLock Durable Object

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⚠️ Definition of Done: this issue must be completed in full, in a single PR. Do not split this
work across multiple PRs, and do not defer any Deliverable below to a follow-up issue. A PR that
satisfies only some of the Deliverables, stubs a required test, or leaves a checkbox
partially-done does NOT resolve this issue and will be closed.

Context

src/env.d.ts:73-75 carries a standing TODO for "a per-PR LOCK Durable Object (SubmissionLock mutex)... deliberately NOT declared in this chunk." The per-PR actuation mutex (guarding maintenance sweep / draft-dodge-close / reopen-reclose racing on the same PR) is currently a "lightweight interim mutex" (src/queue/transient-locks.ts:1-16) built on a fail-open cache primitive, used at src/queue/processors.ts:2443. This is real, acknowledged deferred work with no tracking issue found in the current open backlog.

Requirements

Implement SubmissionLock as a Durable Object class with its own migration tag in wrangler.toml/wrangler.jsonc, and wire claimTransientLock/releaseTransientLockIfOwner call sites in transient-locks.ts to prefer it when available, falling back to the current cache-based mutex for self-host installs without Durable Objects.

Deliverables

• A SubmissionLock Durable Object class implementing the per-PR mutex, with its own wrangler.toml/wrangler.jsonc migration tag
• transient-locks.ts's claimTransientLock/releaseTransientLockIfOwner prefer the Durable Object when available, falling back to the existing cache-based mutex otherwise
• A test exercising two concurrent lock-claim attempts against the Durable Object, asserting only one succeeds
• A test confirming the cache-based fallback still works unchanged for self-host installs without Durable Objects configured

All of the above Deliverables are required in the same PR unless the deliverable text itself states otherwise.

Test Coverage Requirements

src/** -- 99%+ patch coverage, branch-counted, on both the Durable Object implementation and the fallback path. All four Deliverables are required in the same PR.

Expected Outcome

The per-PR actuation mutex is backed by a real, strongly-consistent Durable Object where available, closing the acknowledged gap between the documented SubmissionLock design and the current best-effort cache mutex, while preserving self-host compatibility.

Links & Resources

• src/env.d.ts:73-75
• src/queue/transient-locks.ts:1-16
• src/queue/processors.ts:2443

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JSO-6175