feat(core): add CheckpointManager for periodic actor state snapshots

Gap analysis item 4.2: Bridge checkpoint infrastructure to the
persistent actor H2K/K2H protocol.

- CheckpointConfig: interval (default 30s), max_snapshots (5), storage path
- SnapshotRequest/SnapshotResponse: backend-agnostic protocol types
  mapping to CUDA H2K SnapshotActor / K2H SnapshotComplete
- CheckpointManager: poll_due_snapshots(), complete_snapshot(),
  load_latest(), retention enforcement, pending tracking
- 15 new tests (26 total checkpoint tests pass)

Usage: manager.poll_due_snapshots() returns requests to send as H2K
commands; manager.complete_snapshot() persists to storage on K2H response.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mivertowski