Add core sync functions for identifier-based purgatory synchronization

Implement sync_identifier_next_url and sync_identifier_from_url functions
that provide the core URL selection and fetch logic for purgatory sync.

sync_identifier_next_url:
- Pure URL selection logic with no side effects
- Filters out our own domain and already-tried URLs
- Respects domain throttling when domain parameter is None
- Can target a specific domain when domain parameter is Some

sync_identifier_from_url:
- Fetches OIDs from a specific URL via the SyncContext
- Tracks request start/completion with ThrottleManager for rate limiting
- Calls process_newly_available_git_data on successful fetch

Also adds get_throttled_domains_with_untried_urls helper for the main
sync loop to know which DomainThrottle queues to enqueue identifiers to.

These functions are designed to be called by both:
- Main sync loop (tries non-throttled URLs immediately)
- DomainThrottle queue processing (when capacity frees up)

Includes 10 unit tests covering:
- Throttled domain skipping
- Tried URL skipping
- Our domain filtering
- Specific domain targeting
- Fetch success/failure handling
- Throttle request tracking

1 files changed, 5 insertions, 0 deletions

diff --git a/src/purgatory/sync/mod.rs b/src/purgatory/sync/mod.rs
index 1ac0cb1..d26c1f0 100644
--- a/src/purgatory/sync/mod.rs
+++ b/src/purgatory/sync/mod.rs
@@ -7,10 +7,15 @@
 //! - Debouncing for burst event arrivals
 
 mod context;
+mod functions;
 mod queue;
 mod throttle;
 
 pub use context::{ProcessResult, SyncContext};
+pub use functions::{
+    get_throttled_domains_with_untried_urls, sync_identifier_from_url, sync_identifier_next_url,
+    ThrottledDomainInfo,
+};
 pub use queue::SyncQueueEntry;
 pub use throttle::{DomainThrottle, ThrottleManager};