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-checkpoint-policy/tests/session-checkpoint-policy.spec.ts

@@ -22,6 +22,9 @@ class TestPersistence extends SessionPersistence {
   inspect(_id: SessionId): Promise<{ meta: SessionHeader; events: SessionEvent[] }> {
     return Promise.reject(new Error('not used'))
   }
+  readFrom(_id: SessionId, _fromSeq: number): Promise<{ meta: SessionHeader; events: SessionEvent[] }> {
+    return Promise.reject(new Error('not used'))
+  }
   list(): Promise<SessionHeader[]> { return Promise.resolve([]) }
   listSnapshots(): Promise<never[]> { return Promise.resolve([]) }
 }