…oss under high load

Under high transaction load, snapshot signatures were being lost because:
- All messages shared a single FIFO queue
- ReqSn/AckSn protocol messages got buried behind ReqTx transactions
- When AckSn arrived before local ReqSn was processed, it got re-enqueued
  to the back of the queue, causing signature collection to fail

Solution: Dual-queue system that processes protocol messages before transactions
- HighPriority: ReqSn, AckSn, ChainInput, ClientInput, ConnectivityEvent
- LowPriority: ReqTx, ReqDec

This ensures protocol state machine messages are never starved by transaction load.

Add configurable batch-size and time-interval based snapshot throttling:

- Add --snapshot-batch-size CLI option (default: 10) to trigger snapshots
  when pending transaction count reaches threshold
- Add --snapshot-interval CLI option (default: 0.1s) to trigger snapshots
  after time interval when transactions are pending
- Track lastSnapshotTime in CoordinatedHeadState for time-based throttling
- Implement hybrid logic: snapshots trigger on EITHER batch size OR time interval
- Add requestedAt timestamp to SnapshotRequestDecided for observability

Backward compatible: set snapshotBatchSize=1 for legacy per-tx behavior.

Files changed:
- Environment.hs: Add snapshotBatchSize/snapshotInterval fields
- Options.hs: Add CLI parsers and defaults
- HeadLogic.hs: Implement batch-size and time-based throttling logic
- State.hs: Add lastSnapshotTime tracking
- Outcome.hs: Add requestedAt to SnapshotRequestDecided
- api.yaml: Update JSON schema with new fields
- Test fixtures and golden files updated accordingly