Fix stack overflow with circular or long dead-letter chains

[spuun]

WHAT is this pull request doing?

Dead-lettering was implemented via direct recursive calls, causing a stack overflow when two queues mutually dead-lettered to each other or when a sufficiently long dead-letter chain was traversed.

Changes:

• Replaced recursive dead-letter routing with a trampoline pattern: the DeadLetterer now collects pending (queue, message, callback) tuples into a Context deque and drains them iteratively, keeping the call stack flat regardless of chain length
• Added a routed callback to DeadLetterer#route so the source message is deleted only after it has been successfully enqueued at the destination (preserving message safety)
• Introduced publish_internal on queues to accept and propagate the dlx_context through publish → drop_overflow → expire_msg → route, avoiding allocating a new context per hop
• Moved drop_overflow calls triggered by policy updates into a spawned fiber to avoid re-entrancy

Supersedes #1721

HOW can this pull request be tested?

Specs