Per-conversation ObservableObjects with O(1) dedup via an incrementally
maintained message-ID index, binary-search timestamp insertion, folded
cap policies, a no-downgrade delivery rule, and a typed change subject.
All mutation flows through store intents (conversation mutators are
fileprivate). The previous store is renamed LegacyConversationStore
pending deletion in step 5. 16 behavioral tests including per-
conversation publish isolation; store.append benchmarks at ~144k
messages/sec.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
