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

internal/photos/bridge.go

@@ -10,11 +10,8 @@ type Bridge interface {
 	ListAlbums() ([]Album, error)
 	ListAssets(albumID string) ([]Asset, int, error)
 	ListTree() ([]CollectionNode, error)
-	ExportAlbumPreviews(albumID, outputDir string, targetSize int) (int, error)
-	ExportAlbumOriginals(albumID, outputDir string) (int, error)
 	ExportPreview(assetID, outputDir string, targetSize, index int) (ExportResult, error)
 	ExportOriginal(assetID, outputDir string, index int) (ExportResult, error)
-	BackupAll(outputDir string, targetSize int, originals bool) (int, error)
 	Cancel()
 }
 
@@ -62,23 +59,3 @@ func ParseExportResultJSON(jsonStr string) (ExportResult, error) {
 	}
 	return resp.ExportResult, nil
 }
-
-func InterpretExportResult(rc int) (int, error) {
-	switch rc {
-	case -1:
-		return 0, fmt.Errorf("invalid arguments or directory creation failed")
-	case -2:
-		return 0, fmt.Errorf("could not create output directory")
-	case -3:
-		return 0, fmt.Errorf("album not found")
-	case -4:
-		return 0, fmt.Errorf("all exports failed")
-	case -5:
-		return 0, fmt.Errorf("cancelled")
-	default:
-		if rc < 0 {
-			return 0, fmt.Errorf("unknown error (code %d)", rc)
-		}
-		return rc, nil
-	}
-}