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/Sheets/ServerIdentitySheet.swift

@@ -0,0 +1,90 @@
+import AppKit
+import VoiceCatCore
+
+final class ServerIdentitySheet: NSViewController {
+
+    var onComplete: ((Bool) -> Void)?
+
+    private let tofuStatus: VoiceCatTofuStatus
+    private let displayFingerprint: String
+
+    init(tofuStatus: VoiceCatTofuStatus, displayFingerprint: String) {
+        self.tofuStatus = tofuStatus
+        self.displayFingerprint = displayFingerprint
+        super.init(nibName: nil, bundle: nil)
+    }
+
+    required init?(coder: NSCoder) { fatalError() }
+
+    override func loadView() {
+        view = NSView(frame: NSRect(x: 0, y: 0, width: 440, height: 220))
+    }
+
+    override func viewDidLoad() {
+        super.viewDidLoad()
+        buildUI()
+    }
+
+    private func buildUI() {
+        let isMismatch = tofuStatus == .mismatch
+        let titleText = isMismatch ? "Server Identity Mismatch — Possible MITM!" : "New Server Identity"
+        let titleLabel = NSTextField(labelWithString: titleText)
+        titleLabel.font = .boldSystemFont(ofSize: 14)
+        if isMismatch { titleLabel.textColor = .systemRed }
+        titleLabel.setAccessibilityLabel(titleText)
+
+        let bodyText: String
+        if isMismatch {
+            bodyText = "The server's identity has changed since you last connected. This may indicate a man-in-the-middle attack or that the server was reinstalled. Do NOT accept unless you know why the identity changed."
+        } else {
+            bodyText = "This is the first time you are connecting to this server. Verify the fingerprint below with the server administrator before accepting."
+        }
+        let bodyLabel = NSTextField(wrappingLabelWithString: bodyText)
+        bodyLabel.textColor = .labelColor
+
+        let fpLabel = NSTextField(labelWithString: "Server fingerprint:")
+        let fpField = NSTextField(labelWithString: displayFingerprint.isEmpty ? "(not available)" : displayFingerprint)
+        fpField.font = NSFont.monospacedSystemFont(ofSize: 11, weight: .regular)
+        fpField.isSelectable = true
+        fpField.setAccessibilityLabel("Server identity fingerprint: \(displayFingerprint)")
+
+        let rejectButton = NSButton(title: "Reject (Disconnect)", target: self, action: #selector(rejectClicked))
+        rejectButton.bezelStyle = .rounded
+        rejectButton.setAccessibilityLabel("Reject server identity and disconnect")
+        if isMismatch { rejectButton.keyEquivalent = "\r" }
+
+        let acceptButton = NSButton(title: "Accept", target: self, action: #selector(acceptClicked))
+        acceptButton.bezelStyle = .rounded
+        if !isMismatch { acceptButton.keyEquivalent = "\r" }
+        acceptButton.setAccessibilityLabel("Accept server identity and continue connecting")
+
+        let buttonRow = NSStackView(views: [NSView(), rejectButton, acceptButton])
+        buttonRow.orientation = .horizontal; buttonRow.spacing = 8
+
+        let fpRow = NSStackView(views: [fpLabel, fpField])
+        fpRow.orientation = .horizontal; fpRow.spacing = 8
+
+        let stack = NSStackView(views: [titleLabel, bodyLabel, fpRow, buttonRow])
+        stack.orientation = .vertical
+        stack.spacing = 12
+        stack.edgeInsets = NSEdgeInsets(top: 20, left: 20, bottom: 20, right: 20)
+        stack.translatesAutoresizingMaskIntoConstraints = false
+        view.addSubview(stack)
+        NSLayoutConstraint.activate([
+            stack.topAnchor.constraint(equalTo: view.topAnchor),
+            stack.leadingAnchor.constraint(equalTo: view.leadingAnchor),
+            stack.trailingAnchor.constraint(equalTo: view.trailingAnchor),
+            stack.bottomAnchor.constraint(equalTo: view.bottomAnchor),
+        ])
+    }
+
+    @objc private func acceptClicked() {
+        dismiss(nil)
+        onComplete?(true)
+    }
+
+    @objc private func rejectClicked() {
+        dismiss(nil)
+        onComplete?(false)
+    }
+}