Projections / Persistence Query — read-side query layer

[pathosDev]

Today Journal.read(pid, fromSeq) lets you scan an event stream once. What's missing is a continuous subscribe — read events as they're appended, materialise read-models, persist offsets so projections survive restart.

Strategy: pull-model first. The first iteration polls the journal at a configurable interval (default 1 s) for new events. Simple, robust, deterministic. Push-model (subscribe via EventStream) follows later when workload demands it.

API addition on the Journal interface:

interface PersistenceQuery {
  eventsByPersistenceId(pid, fromSeq): AsyncIterator<PersistentEvent>;
  eventsByTag(tag, fromOffset): AsyncIterator<PersistentEvent>;
}

ProjectionActor lifecycle:

• preStart: load offset from offset-store (SQLite / DurableStateStore).
• onReceive: pump events from the query iterator, run user handler, advance offset.
• Idempotency: handler must be safe to call twice for the same event (event re-delivery is allowed).
• Restart: pick up from last persisted offset.

Components:

File	Task
src/persistence/query/PersistenceQuery.ts (new)	interface
src/persistence/query/InMemoryQuery.ts (new)	for InMemoryJournal
src/persistence/query/SqliteQuery.ts (new)	WHERE seq > ? OR tags LIKE '%t%'
src/persistence/query/CassandraQuery.ts (new)	token-range scan + tags secondary index
src/persistence/projection/ProjectionActor.ts (new)	actor wrapper + offset-store + at-least-once delivery
tests/unit/persistence/projection/*.test.ts (new)	round-trip, restart, idempotency tests
examples/persistence/projection-bank-statement.ts (new)	materialises account histories from existing bank-account events

Estimate: 4-6 days. Orthogonal to #34 / #35 — can land in parallel with the cluster work.

Verification:

• 4 tests: round-trip, tag filter, restart-from-offset, concurrent-writers-don't-deadlock.
• Example: bank-statement projection over examples/persistence/bank-account.ts.

Out of scope (for v1): push-based subscribe (would integrate with EventStream), per-event delivery guarantees beyond at-least-once, projection restart from arbitrary point in time.

See the roadmap plan for full context (item 4 of 5).

[pathosDev]

Closed by a1b95b2.

Delivered:

• src/persistence/query/PersistenceQuery.ts — interface + composite Offset (timestamp + pid + seqNr) so two events sharing a Date.now() tick still sort deterministically.
• src/persistence/query/InMemoryQuery.ts — reference implementation that walks any Journal.read. Correct for every backend, fast for the in-memory one.
• src/persistence/query/SqliteQuery.ts — overrides currentEventsByTag with a real SQL WHERE ',' || tags || ',' LIKE '%,foo,%' so the scan stays in the DB; the per-pid path inherits journal walking.
• src/persistence/query/CassandraQuery.ts — deliberately inherits journal walking. The default Cassandra schema has no secondary index on tags; the header comment spells out exactly when to subclass and what side table to maintain.
• src/persistence/projection/OffsetStore.ts — InMemoryOffsetStore + DurableStateOffsetStore (wraps any DurableStateStore).
• src/persistence/projection/ProjectionActor.ts — actor wrapper with at-least-once delivery (load → handle → save → repeat), exposed via two static factories byPersistenceId / byTag.

Tests (18 new, all green):

• tests/unit/persistence/query/PersistenceQuery.test.ts (11) — comparator, round-trip, tag filter (incl. SQLite false-positive guard for tag substrings), live-stream cancellation.
• tests/unit/persistence/projection/ProjectionActor.test.ts (7) — round-trip, restart-from-offset, idempotency-after-throw, tag filter, DurableState resumption, concurrent writers.

Example:

• examples/persistence/projection-bank-statement.ts — materialises per-account ledgers from events emitted by bank-account.ts.

Verification (matches the issue's checklist):

• ✅ round-trip
• ✅ tag filter
• ✅ restart-from-offset
• ✅ concurrent-writers-don't-deadlock
• ✅ bank-statement example

Out of scope (deferred):

• Push-based subscribe via EventStream (issue body explicitly defers this).
• Restart from arbitrary point in time.
• Per-event delivery guarantees beyond at-least-once.

Suite: 1130 pass / 10 skip / 0 fail (was 1112).