Add thread_local HashSet<ObjRef> tracking for circular reference detection
in the Object Parser. This prevents infinite recursion when PDF objects
contain circular references.
- Created cycle.rs module with RESOLVING thread_local storage
- ResolutionGuard RAII ensures cleanup on drop (even on panic)
- is_resolving() helper for cycle detection
- All 13 cycle tests pass
Closes: pdftract-522li
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
