[AZ-651] [AZ-668] lost-link failsafe ladder + mapobjects persistence (batch 7)

AZ-651 (mission_executor lost-link ladder):
- LostLinkLadder pure-logic state machine (LinkOk -> Degraded -> Lost
  -> LinkLostInFollow + MavlinkLost branch). Configurable thresholds
  via LostLinkConfig.
- LostLinkCommandIssuer trait + MavlinkCommandIssuer production impl
  emitting MAV_CMD_NAV_RETURN_TO_LAUNCH via MavlinkHandle::send_command.
- LostLinkDriver task wires the ladder to operator-link watch, MAVLink
  LinkEvent broadcast, and optional target-follow signal. On RTL,
  driver calls the issuer THEN MissionExecutorHandle::failsafe_trigger.
- failsafe_trigger(LinkLost | LinkLostInFollow) short-circuits FlyMission
  -> Land via direct FSM state mutation + TransitionEvent emission;
  Paused state is intentionally NOT overridden.
- Tests: 4/4 ACs locally green (degraded-no-rtl; lost-fires-once;
  follow-grace; mavlink-loss-no-rtl) plus driver + FSM integration.

AZ-668 (mapobjects_store persistence):
- Snapshot serializable shape + Store::{to_snapshot,from_snapshot}
  round trip.
- MapObjectsPersistence async trait + JsonSnapshotEngine default impl
  (write to .tmp, sync_all, atomic rename, best-effort parent fsync).
- PersistenceError::{Corrupt, SchemaMismatch} surfaces explicit errors
  on bad blob; PersistenceMetrics tracks last_snapshot_ts,
  snapshot_size_bytes, snapshot_errors_total.
- MapObjectsStore::from_snapshot factory for crash recovery from the
  composition root.
- Tests: 4/4 ACs locally green (round-trip; atomic rename ignores
  partial .tmp; crash recovery preserves pending; corruption returns
  explicit error) plus schema-mismatch + metrics smoke checks.

Quality gates:
- cargo fmt: clean.
- cargo clippy -p mission_executor -p mapobjects_store --tests: 0 warns.
- cargo test --workspace: all green.

Co-authored-by: Cursor <cursoragent@cursor.com>

Cargo.lock

@@ -1220,11 +1220,13 @@ dependencies = [
 name = "mapobjects_store"
 version = "0.1.0"
 dependencies = [
+ "async-trait",
  "chrono",
  "h3o",
  "serde",
  "serde_json",
  "shared",
+ "tempfile",
  "thiserror 1.0.69",
  "tokio",
  "tracing",