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

docs/adr/001-cloud-identifier.md

@@ -0,0 +1,21 @@
+# ADR 001: Cloud Status Detection via performSelector
+
+## Status
+
+Accepted
+
+## Context
+
+We need to detect whether a photo asset is stored locally or in iCloud. Apple's PhotoKit does not expose `PHAsset.cloudIdentifier` as a public property on macOS. The `PHAsset` class has this property on iOS but it is undocumented on macOS.
+
+## Decision
+
+Use `performSelector:@selector(cloudIdentifier)` with `@try/@catch` to detect cloud status. If the selector returns a non-null value, the asset is in iCloud. If it throws an exception, fall back to checking `PHAssetResource.isLocallyAvailable` (also via `performSelector`).
+
+## Consequences
+
+- This accesses an undocumented Apple API. It may break in any macOS update without warning.
+- The `@try/@catch` pattern prevents crashes if the selector is removed.
+- If both checks fail or throw, we default to "local" — this may incorrectly report cloud-only assets as local.
+- This approach could cause notarization issues if Apple enforces stricter private API checks in the future.
+- No alternative public API exists on macOS for this purpose as of macOS 14.