Add DomainThrottle for per-domain rate limiting

Implement per-domain throttling for purgatory sync operations:
- Concurrent request limit (max in-flight requests per domain)
- Rate limit (max requests per minute via sliding window)
- Fair round-robin queue processing across identifiers
- In-progress tracking to prevent duplicate fetches
- Tried URL tracking per identifier

Add indexmap dependency for ordered iteration in round-robin queue.

Includes 6 unit tests covering:
- Concurrent limit enforcement
- Rate limit enforcement (sliding window)
- Round-robin fair processing
- In-progress identifier skipping
- Round-robin index adjustment on removal
- Tried URL merging on re-enqueue

1 files changed, 2 insertions, 0 deletions

diff --git a/src/purgatory/sync/mod.rs b/src/purgatory/sync/mod.rs
index 7b6d64a..3c0d875 100644
--- a/src/purgatory/sync/mod.rs
+++ b/src/purgatory/sync/mod.rs
@@ -7,5 +7,7 @@
 //! - Debouncing for burst event arrivals
 
 mod queue;
+mod throttle;
 
 pub use queue::SyncQueueEntry;
+pub use throttle::DomainThrottle;