feat(session-persistence): readFrom(seq) primitive for watermark tail reads

SessionPersistence grows readFrom(id, fromSeq, signal?): the non-mutating
read-from-seq primitive for checkpoint consumers (the persisted projection
cache folds only the tail past its watermark). Coordinator owns validation,
per-id serialization, and the sequential fallback (loadStored + forward
skip); SQLite implements the optional seek-capable loadStoredFrom hook
(WHERE seq >= ?), JSONL stays sequential by contract. Contract suite covers
suffix exactness, empty-tail, non-mutation, and cancellation; seam README
(both languages) documents the method and the hook.

packages/session-query/session-query-sqlite/tests/sqlite.spec.ts

@@ -149,6 +149,11 @@ class TestPersistence extends SessionPersistence {
     return structuredClone(entry)
   }
 
+  async readFrom(id: SessionIdType, fromSeq: number, signal?: AbortSignal): Promise<{ meta: SessionHeader; events: SessionEvent[] }> {
+    const whole = await this.inspect(id, signal)
+    return { meta: whole.meta, events: whole.events.filter(event => event.seq >= fromSeq) }
+  }
+
   async list(): Promise<SessionHeader[]> {
     TestPersistence.listStarted?.()
     await TestPersistence.listGate