feat(macos): VoiceCatMac AppKit client + fix xcodebuild

The previous "shipped" claim was false — xcodebuild had never been run and
the macOS app source was never committed. This commit adds the 17 Swift
source files + xcodeproj and fixes three real defect classes so Debug and
Release both build clean:

1. MainWindowController.swift compile errors:
   - NSAccessibility.post arg order (element:notification:userInfo:)
   - NSAccessibilityPriorityMedium -> NSAccessibilityPriorityLevel.medium
   - StreamSummary.streamId -> .id (Identifiable conformance)
   - drop redundant VoiceCatResult.description extension
2. Linker: add -lc++ to OTHER_LDFLAGS (libvoicecat-fat.a is C++20; pure-Swift
   app target has no .cpp sources so libc++ wasn't pulled in — swift test
   passed because Package.swift testTarget has linkerSettings: c++).
3. Release config: add ONLY_ACTIVE_ARCH=YES (XCFramework only has arm64).

Verified: clean Debug + Release builds, otool -L shows libc++.1.dylib,
nm shows _vc_client_create/_vc_version_string, app launches and runs.

CLAUDE.md

@@ -6,9 +6,9 @@ and what's next* read [`PROGRESS.md`](PROGRESS.md); for *design* read [`docs/`](
 
 > **One-line status:** M5 (moderation & admin UI) is complete — permissions, kick/ban/move,
 > server-mute, channel CRUD, in-app account management, disconnect/keepalive/reaper. Windows
-> WinForms C# client is shipped (M4). **macOS port validated** — `dev` + `apple-dev` presets
-> build green, 21/21 tests pass on macOS. `ctest --preset dev` green — 21/21 tests.
-> macOS/iOS Swift client is next. See [`PROGRESS.md`](PROGRESS.md).
+> WinForms C# client is shipped (M4). **macOS AppKit client shipped** — `VoiceCatMac.xcodeproj`
+> at `clients/apple/macOS/`. `ctest --preset dev` green — 21/21 tests.
+> iOS SwiftUI client is next. See [`PROGRESS.md`](PROGRESS.md).
 
 VoiceCat = self-hosted native voice & text chat (TeamSpeak/Mumble-style). Plain TCP (control)
 + UDP (media), no WebRTC, encrypted by default. A shared C++ core (`libvoicecat`) drives