fix(acp): add independent turn watchdog and close 15 follow-up findings

[michaelzeyuchen]

Summary

Two related hardening changes to buzz-acp, extracted from a longer fork
branch onto current main.

1. Independent turn watchdog. A managed-agent turn that stops producing
output without terminating currently holds its pool slot indefinitely — the
existing timeouts cover idle input, not a wedged in-flight turn. This adds a
watchdog armed per turn and disarmed on completion, so an abandoned turn is
aborted, its in-flight entry released, and its slot respawned rather than
leaked.

2. Fifteen follow-up fixes found while reviewing the above:

• observer publish queue is byte-bounded rather than count-bounded, and drops
  oldest on overflow instead of growing without limit
• requeue preserves timestamps and cancellation state, with an explicit cap on
  the cancelled side-store
• drain tests exercise the real drain paths instead of reimplementing them
• respond_to gate is case-insensitive
• session-time helpers consolidated behind the idle-timeout guard
• lazy_pool reported in the summary
• incremental batch budget matches exhaustive serialization
• BUZZ_AUTH_TAG parse failure warns instead of failing silently
• is_dm_channel resolves in the background
• BUZZ_ACP_EVENT_BUFFER=0 falls back to the default

Testing

From a clean checkout of this branch, with the Hermit toolchain active:

• cargo fmt --check — clean
• cargo clippy --workspace --all-targets --all-features -- -D warnings — clean
• cargo test -p buzz-acp — 841 passed, 0 failed

That includes watchdog_abort_releases_inflight_and_respawns_slot, which pins
the watchdog behaviour described above.

Notes for reviewers

• The [skip-ultra-ship] tokens in the commit subjects are an artifact of a
  local pre-commit tool and carry no meaning upstream. Happy to reword or
  squash if preferred.
• GH#5439: Follow the user's threading choice for managed-agent replies #5440 also touches crates/buzz-acp (queue.rs) for managed-agent
  threading. The overlap looks small, but flagging it so whichever lands second
  can be rebased deliberately rather than hitting a surprise conflict.