Replace the SharedFlow-based broadcast in LiveEventStore with an
inverted index over filter-bearing subscribers. Each REQ registers
its filters into a per-store FilterIndex<LiveSubscription>; insert()
calls index.candidatesFor(event) and only delivers to candidates
whose Filter.match still passes. Cuts the per-event walk from
O(N_subs * N_filters) to a few hash lookups plus match() over a
small candidate set.
FilterIndex itself lives next to Filter.kt (commonMain, KMP-friendly,
AtomicReference + COW) so other call sites with the same shape
(LocalCache.observables, ObservableEventStore.changes) can reuse it.
Each filter contributes entries on its single most-selective dimension
(ids > authors > tags > tagsAll > kinds > unindexed) to keep buckets
narrow and avoid Set-dedupe work in candidatesFor.
The historical-replay race the previous SharedFlow + onSubscription
handoff closed is preserved by registering BEFORE replay starts and
deduping seen ids until EOSE.
Claude
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