refactor(claude): queue owns its done latch + steering Event (C4)

[TylerLeonhardt]

Stacked on #326270 (C9). Review that first; this PR's diff is the single commit on top. Base retargets as the stack lands.

What

C4 — delete the last two back-reference closures. ClaudeSdkPipeline injected two closures into ClaudePromptQueue: () => this._abortController.signal (read on every next()) and (pendingId) => this._onDidProduceSignal.fire(...) (steering). These are the "closures passed down that call instance functions" from the original review. C4 removes both, so the queue holds no back-reference into the pipeline.

• Abort → the queue owns a terminal _done latch, set by notifyAborted() (which the pipeline already calls after aborting its controller). next() checks the latch instead of reaching back through a () => signal closure into the pipeline's swap-prone field.
• Steering → onDidYieldSteering: Event<string> replaces the injected callback; the pipeline subscribes, matching the existing router→pipeline Event pattern.
• resetForRebind deleted — dead since C9 made the pipeline immutable (a rebuild mints a fresh queue rather than reviving one), so the latch never needs clearing. This is why C4 stacks naturally on C9: the immutability removed the only reason the latch would ever reset.
• _wireAbortHandler now also handles an already-aborted controller directly (addEventListener('abort') doesn't fire retroactively); the queue is constructed before wiring so the latch is reachable.

The queue ctor drops from 4 args to 2. Net −30 lines.

Why it's safe

• _done faithfully models "controller aborted": every abort path (session.abort, pipeline.abort, shutdownAndWait, dispose) aborts the controller → fires _wireAbortHandler → notifyAborted() → _done. The queue's termination no longer depends on reading an external signal that could go stale.
• Steering behavior is unchanged — steering_consumed still fires the moment a steering entry is handed to the SDK, now via the Event.

Validation

• Suites green: claudePromptQueue 12, claudeSdkPipeline 12, claudeAgent 197.
• typecheck-client / eslint / valid-layers-check clean.
• The abort/steering/parked-next() semantics are pinned by claudePromptQueue.test.ts; the end-to-end abort + steering paths by claudeAgent.test.ts (incl. this stack's C9 abort/rebuild-race edge tests).

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