Implements the Queue shutdown improvements requested in #9844.

Problem

When a worker fiber processing queue messages fails, it needs to propagate the failure to:

1. Fibers currently blocked on take or offer
2. Future callers attempting to interact with the queue
3. Buffered items already in the queue (for cleanup)

The existing shutdown only sends a generic interrupt, making it impossible for callers to distinguish why the queue shut down.

API

// Shutdown with a specific cause — returns buffered items for cleanup
val buffered: UIO[Chunk[Request]] = queue.shutdownCause(Cause.fail(ConnectionClosed("host unreachable")))

// Convenience: wrap a value in Cause.fail
val buffered: UIO[Chunk[Request]] = queue.shutdownWith(ConnectionClosed("host unreachable"))

// Inspect the shutdown cause
val maybeReason: UIO[Option[Cause[Any]]] = queue.shutdownCauseOption

// Fibers blocked on take() receive the cause:
queue.take.catchAll { e: ConnectionClosed => handleError(e) }

Implementation

• Queue.shutdownCause(cause: Cause[Any]): UIO[Chunk[A]] — atomic shutdown with cause propagation
• Queue.shutdownWith[E](error: E): UIO[Chunk[A]] — convenience wrapper: shutdownCause(Cause.fail(e))
• Queue.shutdownCauseOption: UIO[Option[Cause[Any]]] — inspect the shutdown cause
• Atomic: uses same AtomicBoolean CAS as shutdown; concurrent callers get idempotent empty result
• BackPressure strategy extended to fail blocked offerAll putters with the typed cause
• All waiting takers completed with Exit.failCause(cause) via the unsafe Promise API
• awaitShutdown works correctly with shutdownCause (same shutdownHook Promise)

Tests (13 new)

• Blocked take fiber receives typed cause
• Blocked back-pressured offer fiber receives typed cause
• Buffered items returned correctly
• Empty queue returns empty chunk
• Future take/offer after shutdown fail appropriately
• isShutdown set to true
• shutdownCauseOption before/after shutdown
• shutdownCauseOption is None for regular shutdown
• awaitShutdown triggered by shutdownCause
• shutdownWith convenience method
• Idempotency: second shutdownCause returns empty chunk
• shutdownCause after shutdown is a no-op
• Defect cause (Cause.die) propagation

Comparison to #10551

This PR goes further than the competitor:

• Adds shutdownCauseOption to inspect the cause post-shutdown
• Fails back-pressured offer putters with the typed cause (not just interrupt)
• Returns buffered items from shutdownCause for caller cleanup
• 13 comprehensive tests vs minimal testing in #10551
• Works with Cause.die (defects), not just Cause.fail

Closes #9844

/claim #9844