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.

clients/apple/Sources/VoiceCatCore/Callbacks.swift

@@ -0,0 +1,51 @@
+// Callbacks — the C function pointers passed to `vc_callbacks`. These are the Swift
+// equivalent of the C# client's `[UnmanagedCallersOnly]` static methods (NativeCallbacks.cs).
+//
+// The critical patterns (carried over from the proven C# implementation):
+//   1. `@convention(c)` closures — plain C function pointers, NOT GC/ARC-managed closures.
+//      A @convention(c) closure cannot capture context, which is why the `user` pointer is
+//      used to resolve back to the VoiceCatClient instance (the C# version uses GCHandle for
+//      the same thing; Swift uses Unmanaged).
+//   2. `Unmanaged.passUnretained(self).toOpaque()` as the `user` context — a stable raw
+//      pointer to the Swift object WITHOUT incrementing the retain count. This is safe
+//      because `deinit` calls `vc_client_destroy` (which synchronously joins every internal
+//      thread) BEFORE the object's memory is freed — so no callback can fire after the object
+//      is gone. (The C# equivalent: GCHandle.Alloc + GCHandle.Free in Dispose.)
+//   3. Copy `ev.text` to a Swift `String` INSIDE `onEvent` (via `VoiceCatEvent.from(_:)`)
+//      before returning — the raw pointer is dangling after the callback returns. This is
+//      the #1 lifetime rule from voicecat.h's vc_event doc comment.
+
+import VoiceCatC
+import Foundation
+
+/// Internal: builds the `vc_callbacks` struct wired to VoiceCatClient's C function pointers.
+/// The `user` context is an Unmanaged-passUnretained pointer to the client — resolved back
+/// to the client inside `onEvent`/`onLevel` below.
+internal enum Callbacks {
+    /// The `on_event` C function pointer. Non-capturing @convention(c) closure — resolves
+    /// the VoiceCatClient from `user` and enqueues a safe copy of the event.
+    static let onEvent: @convention(c) (
+        UnsafeMutableRawPointer?, UnsafePointer<vc_event>?
+    ) -> Void = { user, ev in
+        guard let user, let ev else { return }
+        let client = Unmanaged<VoiceCatClient>.fromOpaque(user).takeUnretainedValue()
+        // Copy the event (including text) to a Swift value NOW — the raw vc_event is
+        // invalid after this callback returns.
+        client.enqueueEvent(VoiceCatEvent.from(ev.pointee))
+    }
+
+    /// The `on_level` C function pointer. Coalesces to "latest sample per stream_id"
+    /// (intermediate values are visually irrelevant — same as C#'s ConcurrentDictionary).
+    static let onLevel: @convention(c) (
+        UnsafeMutableRawPointer?, UInt32, Float
+    ) -> Void = { user, streamId, rms in
+        guard let user else { return }
+        let client = Unmanaged<VoiceCatClient>.fromOpaque(user).takeUnretainedValue()
+        client.enqueueLevel(streamId, rms)
+    }
+
+    /// Construct the vc_callbacks struct for a given client.
+    static func make(user: UnsafeMutableRawPointer) -> vc_callbacks {
+        vc_callbacks(on_event: onEvent, on_level: onLevel, user: user)
+    }
+}