Edison Jwa
82ea1a117f
feat(core): FileTransferService with coalescing, throttling, negative cache
- New file_transfer module with FileTransferService struct
- Semaphore(2) throttles concurrent downloads
- In-flight HashMap coalesces duplicate avatar requests
- 5-min negative cache short-circuits ServerRejected misses
- ChanoraSession delegates get_avatar through the service
- connect/disconnect update shared protocol handle
- clear_cache/cache_size delegate to service
- 2 new unit tests (cached hit, negative cache)