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.

clients/apple/macOS/VoiceCatMac/AppDelegate.swift

@@ -0,0 +1,38 @@
+import AppKit
+
+final class AppDelegate: NSObject, NSApplicationDelegate {
+    private var connectWindowController: ConnectWindowController?
+
+    func applicationDidFinishLaunching(_ notification: Notification) {
+        buildMenuBar()
+        NSApp.setActivationPolicy(.regular)
+        connectWindowController = ConnectWindowController()
+        connectWindowController?.showWindow(nil)
+        NSApp.activate(ignoringOtherApps: false)
+    }
+
+    func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool { true }
+
+    private func buildMenuBar() {
+        let mainMenu = NSMenu()
+
+        let appItem = NSMenuItem()
+        mainMenu.addItem(appItem)
+        let appMenu = NSMenu()
+        appMenu.addItem(NSMenuItem(title: "Quit VoiceCat",
+                                   action: #selector(NSApplication.terminate(_:)),
+                                   keyEquivalent: "q"))
+        appItem.submenu = appMenu
+
+        let editItem = NSMenuItem()
+        mainMenu.addItem(editItem)
+        let editMenu = NSMenu(title: "Edit")
+        editMenu.addItem(NSMenuItem(title: "Cut", action: #selector(NSText.cut(_:)), keyEquivalent: "x"))
+        editMenu.addItem(NSMenuItem(title: "Copy", action: #selector(NSText.copy(_:)), keyEquivalent: "c"))
+        editMenu.addItem(NSMenuItem(title: "Paste", action: #selector(NSText.paste(_:)), keyEquivalent: "v"))
+        editMenu.addItem(NSMenuItem(title: "Select All", action: #selector(NSText.selectAll(_:)), keyEquivalent: "a"))
+        editItem.submenu = editMenu
+
+        NSApp.mainMenu = mainMenu
+    }
+}