v0.2.0: semaphore timeouts, error logging, dead code removal, parallel exports

Critical:
- Replace DISPATCH_TIME_FOREVER with 120s/30s timeouts in ObjC
- Log failed asset IDs and error messages in cmdExport/backupTree
- Show failed count in export summaries

Cleanup:
- Remove legacy Bridge methods (ExportAlbumPreviews, ExportAlbumOriginals, BackupAll)
- Remove legacy ObjC functions and C stub equivalents
- Remove photos.go delegates (package-level pass-throughs)
- Remove InterpretExportResult (only used by legacy methods)
- Clean up mockBridge fields (rename Fn2 -> Fn)
- Fix rc race condition in main_main.go (atomic.Int32)
- Remove unused variables (_ = grandTotal, _ = sig)

Design:
- Fix resolveAlbumID: ListAlbums first (cheap), then direct ID
- Unify Cloud type: Asset.Cloud string (was bool)
- Extract shared export logic into exportAssets/exportOne
- Add worker pool for parallel exports (3 workers when assets >= 4)
- Fix backupTree progress bar counter and directory prefix

Robustness:
- Add nil checks for stringWithUTF8String: in ObjC
- Log directory creation errors in ensure_directory (ObjC)

Quality:
- Add go vet and -race flag to Makefile test target
- Add ADR for performSelector cloudIdentifier decision
- Add sync comments between Go/ObjC sanitizePathComponent
- Add package-level doc comment
- Add tests: partial failure, skipped album, album-not-found message

bridge/photokit_bridge.h

@@ -26,23 +26,6 @@ char *photos_export_original_json(
 
 char *photos_list_tree_json(void);
 
-int photos_export_album_previews(
-    const char *album_id,
-    const char *output_dir,
-    int target_size
-);
-
-int photos_export_album_originals(
-    const char *album_id,
-    const char *output_dir
-);
-
-int photos_backup_all(
-    const char *output_dir,
-    int target_size,
-    int originals
-);
-
 void photos_request_cancel(void);
 
 void photos_free_string(char *value);