Proposal: Extract session persistence out of SessionManager

[rushidesai]

Currently SessionManager is tied to file system based persistence. If we could extract that functionality out, it would enable external session persistence (for example in a SQL database). Roughly the design would look like this.

1. Expose SessionStore interface:

export interface SessionStore {
  /** Called when a new session is created. Returns an opaque session ref
   *  (e.g. file path, DB row ID) for callers that need it. */
  initSession(header: SessionHeader): string | undefined;

  /** Write all entries at once — used for initial lazy flush and after migration. */
  flush(header: SessionHeader, entries: FileEntry[]): void;

  /** Append a single entry after the initial flush. */
  appendEntry(entry: SessionEntry): void;

  /** Load all entries from a previously persisted session ref. */
  load(ref: string): FileEntry[];

  /** Materialize a branched session from an explicit entry list.
   *  Returns the new ref, or undefined if stateless. */
  createBranch(newHeader: SessionHeader, entries: FileEntry[]): string | undefined;
}

2. Implement NullSessionStore and FileSystemSessionStore

3. Replace persist flag with session store.