Add ThrottleManager for cross-domain rate limiting

Implements ThrottleManager which manages all per-domain DomainThrottle
instances and provides:

- Throttle status checking via is_throttled() for sync URL selection
- Request tracking via start_request()/complete_request()
- Identifier queue management via enqueue_identifier()
- Automatic domain throttle creation on first access
- Thread-safe access via DashMap with Mutex-wrapped throttles

The manager uses the configured max_concurrent and max_per_minute limits
for all domains. Trigger-based queue processing (set_context,
process_queued_identifier) will be added after SyncContext is available.

Tests verify:
- is_throttled reflects domain capacity correctly
- enqueue_identifier creates domain throttle if needed
- start_request creates domain throttle if needed

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