feat(apple/input): desktop (absolute) mouse mode on macOS — remote-desktop sweep M1

Folds the parked client-side-cursor machinery (cursorMode auto/always/
never, hidden while disabled) into the cross-client mouse model:
MouseInputMode capture|desktop under DefaultsKey.mouseMode, picked in
Settings ▸ Keyboard & mouse (macOS), resolved at session start and
gated off on gamescope hosts (relative-only EIS).

Desktop model = the un-neutered absolute path with the SDL cursor
policy: pointer never disassociated (enters/leaves the stream freely),
monitor forwards letterboxed absolute positions, and the local cursor
hides only while over the view via an invisible-cursor rect (the
host's composited cursor is the one you see; AppKit manages the rect,
so no hide/unhide balancing). ⌘⇧C becomes ⌃⌥⇧M — the same chord as
the SDL clients — flipping the model live with an atomic
release/re-engage.

Verified: swift build + full test suites green (macOS arm64).

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

clients/apple/Sources/PunktfunkKit/Input/MouseInputMode.swift

@@ -0,0 +1,12 @@
+/// How a physical mouse drives the host — the cross-client mouse model (the SDL clients'
+/// `MouseMode` / `Settings::mouse_mode`, design/remote-desktop-sweep.md M1). Stored stringly
+/// under `DefaultsKey.mouseMode`.
+public enum MouseInputMode: String, CaseIterable, Sendable {
+    /// Pointer capture (disassociated, hidden cursor, relative deltas) — the game model,
+    /// and the default: the only cursor you see is the host's.
+    case capture
+    /// Absolute pointer, uncaptured: the cursor enters and leaves the stream freely and
+    /// motion is forwarded as absolute positions through the letterbox. The remote desktop
+    /// model. Requires a host injector with absolute support (not gamescope).
+    case desktop
+}