The previous design wrapped a mutable HashSet in an AtomicReference,
which makes the *reference* thread-safe but not the set's internals.
The historical-replay closure ran `seenIds.load()?.add(event.id)`
from the subscriber's coroutine while `deliver` ran
`seen.contains(event.id)` from the publisher's coroutine —
concurrent add/contains can corrupt buckets or throw CME.
The pre-index implementation didn't have this race because both
operations ran on the same Flow collector coroutine. Index-driven
dispatch put them on different coroutines.
Switch to AtomicReference<Set<String>?> over an immutable Set with a
CAS-loop on add. Reads in `deliver` always see a fully-constructed
snapshot. Single-writer in steady state so the CAS typically
succeeds first try; the loop only matters across the
`seenIds.store(null)` post-EOSE handoff.
Also:
- hoist `NostrSignerSync(keys[target])` out of the per-iteration
loop in LoadBenchmark.fanoutScaling.
- add FilterIndex tests for `tagsAll` selection, multi-char tag
fall-through, and re-register key-union semantics.
Claude
70afcd10ca