feat(bridge): protocol 6 — an idempotency key, and champ.boss.killed (Phase 11a)

A command carrying an `idempotencyKey` is now executed at most once: a repeat is
answered with the original reply rather than re-run. That is the precondition
every world verb in Phase 12 is waiting on, and it is what let `uo.broadcast`
stop being un-retryable.

The gate sits in BridgeBoot's inbound dispatch, not in each handler, so it covers
every kind including ones a later protocol adds. A command with no key behaves
exactly as it did before, which leaves the admin screens unchanged.

Four rules, each a decision rather than an implementation detail: reserve on
receipt (so a handler that defers is covered, answering `bridge.busy` to a repeat
in flight); a key that has begun is never released, not even when the handler
throws; a replay is stamped with the REPEAT's correlation id, because the
sidecar's reqId is fresh per call and replaying the original would hang the retry;
and the bound is loud, because an evicted key is the guarantee's one hole.

`champ.boss.killed` rides along because a bump costs a release, a bundle and an
operator update on every shard. It fires from EventSink.CreatureDeath, detected
by type so a boss that popped and died inside one sweep is still reported, and it
carries the damage table that exists at the death and nowhere else.

overlay.toml protocol = 6, in this commit rather than a later one.

Co-Authored-By: Claude <noreply@anthropic.com>

overlay.toml

@@ -23,8 +23,8 @@
 # manual duty: when the protocol changes, bump it here in the same PR that
 # changes the emitters, exactly as link bumps PROTOCOL_VERSION.
 #
-# Current: 5 — see docs/link/v5.md (house.decay scheduling, vendor.listing fees, account.login.result).
-protocol = 5
+# Current: 6 — see docs/link/v6.md (idempotency keys on inbound commands, champ.boss.killed).
+protocol = 6
 
 # ── ServUO compatibility ─────────────────────────────────────────────────────
 #