feat(apple): VoiceCatCore Swift package + XCFramework build for macOS/iOS clients

Lays the groundwork for the macOS (AppKit) and iOS (SwiftUI) clients with a shared
Swift core wrapping the C ABI, mirroring the proven Windows VoiceCat.Interop layer.

Architecture decision: macOS UI = AppKit (not SwiftUI) for the most mature VoiceOver
accessibility story — same rationale as the Windows client's WinForms-over-WinUI-3
decision. iOS stays SwiftUI. Recorded in docs/roadmap.md §2.

Build infrastructure (Phase 0):
- clients/apple/scripts/build-xcframework.sh: runs cmake --preset apple-dev, merges
  libvoicecat.a + 107 vcpkg static deps into a single ~30 MB fat static library
  (libvoicecat-fat.a) via libtool -static (SPM binary targets link one .a per slice),
  stages voicecat.h + a generated module.modulemap (module VoiceCatC) into the headers,
  runs xcodebuild -create-xcframework -> clients/apple/VoiceCatCore.xcframework.

VoiceCatCore Swift Package (Phase 1):
- Package.swift: binary target (VoiceCatCoreXCF) + library (VoiceCatCore) + test target.
- Sources/VoiceCatCore/: 7 files mirroring the C# VoiceCat.Interop patterns adapted to
  Swift native C interop — Enums (9 Swift mirrors of C enums, UInt32-backed), Config,
  Event (copies ev.text to String inside the callback — the #1 lifetime rule), Models
  (10 Swift value types), Marshaling (C arrays -> Swift + immediate vc_free_*),
  Callbacks (@convention(c) + Unmanaged.passUnretained, the Swift analog of C#'s
  [UnmanagedCallersOnly] + GCHandle), VoiceCatClient (owns vc_client* as OpaquePointer,
  all 38 C ABI functions, deinit -> vc_client_destroy then frees config CStrings, event
  delivery on main queue via coalesced DispatchQueue.main drain).

Tests — 6/6 green (swift test against a real voicecat-server):
- testConnectTofuAuthListChannelsRoundTrips, testAdminChannelCrudAccountCrudRoundTrips,
  testScreenAudioStreamStartsAndStops, testPerStreamRecvControlsRoundTrip, plus two
  static smoke tests. Catches Swift-specific interop bugs (@convention(c) callback
  lifetime, Unmanaged pointer resolution, CString memory management, enum raw-value
  bridging, struct layout) that C++ ctest cannot. C++ suite still 21/21 green.

Docs updated (house rule): tech-stack.md §2, architecture.md §4, roadmap.md M4 + §2,
clients/apple/README.md (full rewrite), PROGRESS.md, .gitignore.

.gitignore

@@ -36,8 +36,12 @@ Thumbs.db
 # Apple / Windows client build artifacts (added in M4)
 clients/apple/**/build/
 clients/apple/**/*.xcodeproj/xcuserdata/
+clients/apple/**/*.xcframework/
 clients/windows/**/bin/
 clients/windows/**/obj/
+# SwiftPM build artifacts
+clients/apple/.build/
+clients/apple/Package.resolved
 
 # Test artifacts: TOFU pin store written by vc_client during headless tests
 # (core/src/core/client.cpp falls back to this relative path when tofu_store_path is unset).