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-persistence/session-persistence-jsonl/src/index.ts

@@ -134,6 +134,12 @@ export class SessionPersistenceJsonl extends SessionPersistence implements Persi
     return this.coordinator.inspect(id, signal)
   }
 
+  // JSONL is sequential media: no loadStoredFrom hook, so the coordinator
+  // parses the stored prefix (both encodings) and skips forward to fromSeq.
+  readFrom(id: SessionId, fromSeq: number, signal?: AbortSignal): Promise<{ meta: SessionHeader; events: SessionEvent[] }> {
+    return this.coordinator.readFrom(id, fromSeq, signal)
+  }
+
   // One method serves both public `list` and the backend hook; delegating it to
   // the coordinator would call this hook recursively.