feat(apple): explicit input-capture state machine — no more cursor grabs on window chrome

Capture used to engage whenever the app became active, so the click that activates the
window — on the title bar (a drag) or a resize edge — got the cursor warped away
mid-gesture, and raw deltas kept streaming to the host while the user fought the window.
Reworked Moonlight-style, with capture as a deliberate, reversible state owned by
StreamLayerView:

- Engage: automatically once when the stream starts / trust is confirmed (one-shot, can
  never fire surprisingly later), or by clicking into the video (that click's
  press/release are suppressed toward the host; acceptsFirstMouse makes it one click
  from another app). NEVER on app re-activation.
- Release: ⌘⎋ (toggles, key-window-scoped), focus loss — now including same-app window
  switches (⌘, / ⌘N / ⌘M resign key without resigning the app; previously the new
  window inherited a hidden frozen cursor and its typing was double-delivered to the
  host) — and disconnect.
- While released: nothing is forwarded (InputCapture.forwarding gates the GC handlers;
  held keys/buttons are flushed host-side so nothing sticks), the cursor is free, and
  the HUD (now showing the capture state) is clickable.
- The no-beep behavior moved from the NSEvent monitor to first-responder key
  consumption — swallowing at the monitor risked starving GC's own delivery (the
  "input broken altogether" report). The monitor now only intercepts ⌘⎋.
- Adversarial-review fixes: a second session preempts the previous one cleanly instead
  of leaving it captured with dead GC handlers (onPreempted); the engage click's
  suppression latch can't outlive the click (mouseUp backstop); ⌘⎋'s physical Esc can't
  type into the host in either toggle direction (suppressedVK latch + Esc-while-⌘
  guard); capture callbacks defer out of the SwiftUI update pass.

Validated live against the box: 16185 input datagrams injected during a captured
session (gamescope EIS), title-bar drag/resize free while released, and visible
cursor + typing on a streamed KWin desktop, all user-confirmed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

clients/apple/Sources/PunktfunkClient/ContentView.swift

@@ -58,7 +58,7 @@ struct ContentView: View {
             return nil
         }()
         return ZStack {
-            stream(capturesCursor: pendingFingerprint == nil)
+            stream(captureEnabled: pendingFingerprint == nil)
                 .blur(radius: pendingFingerprint != nil ? 32 : 0)
                 .overlay {
                     if pendingFingerprint != nil {
@@ -257,19 +257,22 @@ struct ContentView: View {
 
     // MARK: - Stream
 
-    private func stream(capturesCursor: Bool) -> some View {
+    private func stream(captureEnabled: Bool) -> some View {
         Group {
             if let conn = model.connection {
                 StreamView(
                     connection: conn,
-                    capturesCursor: capturesCursor,
+                    captureEnabled: captureEnabled,
+                    onCaptureChange: { [weak model] captured in
+                        model?.mouseCaptured = captured
+                    },
                     onFrame: { [meter = model.meter] au in meter.note(byteCount: au.data.count) },
                     onSessionEnd: { [weak model] in
                         Task { @MainActor in model?.sessionEnded() }
                     }
                 )
                 .overlay(alignment: .topTrailing) {
-                    if capturesCursor { hud(conn) }
+                    if captureEnabled { hud(conn) }
                 }
             }
         }
@@ -279,8 +282,13 @@ struct ContentView: View {
         VStack(alignment: .trailing, spacing: 4) {
             Text("\(conn.width)×\(conn.height)@\(conn.refreshHz)  \(model.fps) fps  \(model.mbps, specifier: "%.1f") Mb/s")
                 .font(.system(.caption, design: .monospaced))
-            // ⌘D because the local cursor is hidden+frozen while streaming — the button
-            // can't be clicked. (Cmd+Tab away also frees the cursor.)
+            // While captured the cursor is hidden+frozen, so the button is keyboard-only
+            // (⌘⎋ or Cmd+Tab release the cursor; released, it's clickable again).
+            Text(model.mouseCaptured
+                ? "⌘⎋ releases the mouse"
+                : "Click the stream to capture input")
+                .font(.caption2)
+                .opacity(0.8)
             Button("Disconnect (⌘D)") { model.disconnect() }
                 .font(.caption)
                 .keyboardShortcut("d", modifiers: .command)