fix(rooms): clear stranded compaction active state at turn settlement

[lsaether]

Addresses item 1 of #66.

Problem

compaction_state.active is set by a Hermes context compression started stderr line and cleared by the matching done. If the done never arrives — the compaction failed, or its line was dropped by the lossy stderr pump (STDERR_CAPACITY) — the flag stayed true indefinitely, so /debug/sessions and session/attach snapshots reported a perpetual "compacting" state to late joiners.

Fix

Hermes compacts within a prompt turn, so by the time that turn's response settles, any compaction it triggered has either completed (cleared by done) or been abandoned. Turn settlement is the bound — the same signal a live client uses to clear its own "compacting" affordance (amux/turn_complete).

• Live path: clear the transient active / pending_hermes_session_id at turn settlement. compaction_count and last_* history are durable and untouched. No frame emitted — live clients already clear on amux/turn_complete.
• Durable path (added after first review): the live clear is in-memory only, so it didn't survive replay-store restart — hydration rebuilds compaction_state from persisted frames, and a persisted context_compaction_started with no done resurrected active = true. Fixed by applying the same bound during hydration: a persisted amux/turn_complete clears a still-active compaction. amux/turn_complete is broadcast (and persisted) on every prompt-turn settlement, so no new marker frame is needed.

Tests

• stranded_compaction_active_clears_at_turn_settlement — live clear on turn settlement, no fabricated count.
• completed_compaction_is_unaffected_by_turn_settlement_clear — happy-path guard.
• hydration_clears_stranded_compaction_via_persisted_turn_complete — started → turn_complete → restart asserts restored active == false (verified it fails without the hydration arm).

cargo test 89 lib + 82 integration green; cargo clippy -D warnings and cargo fmt --check clean.

Scope note

Closes out the actionable part of #66. The other two items are accepted/deferred — rationale in this comment on #66:

• Item 2 (future double-rotation when Hermes ships _meta.hermes) — contingent on a structured upstream signal not being pursued while we support today's Hermes.
• Item 3 (stderr/stdout ordering race) — inherent, bounded, documented in code.

#66 stays open until this merges; I'll close it then.

Refs #66.