feat(input): #89 cursor capture — Input.cursor_mode (hide/lock/confine)

A windowed action game can hide the OS cursor and lock/confine the mouse to the
window. Input.cursor_mode(mode): 0 normal, 1 hidden (draw your own reticle),
2 locked (hidden + dissociated — the mouse feeds relative motion via
Input.mouse_dx/dy and Input.mouse_x/y is a clamped virtual cursor, FPS/twin-stick
aim), 3 confined (dissociated but visible; the mouse can't leave the window).
The platform auto-releases (shows + reconnects) while the window is not key
(Cmd-Tab) and on close, so the cursor is never left captured. Headless it is a
no-op (DCE'd).

Native macOS impl in cocoa.ll: [NSCursor hide]/[unhide] (ref-counted, toggled
only on change so the count stays balanced across focus changes),
CGAssociateMouseAndMouseCursorPosition, and CGGetLastMouseDelta for the relative
virtual cursor, behind a new win_cursor_mode intrinsic. Windowed-only behaviour
(not in the headless golden suite); example compiles headless and links
windowed. Full suite 115/0, fixpoint holds.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

selfhost/backend/emit_head.ludic

@@ -68,6 +68,7 @@ function emit_header() -> void {
   emith("declare void @win_mouse(ptr)\n")
   emith("declare void @win_pad(ptr)\n")
   emith("declare void @win_touch(ptr)\n")
+  emith("declare void @win_cursor_mode(i32)\n")
   emith("declare ptr @snd_load(ptr)\n")               # #22 audio backend (audio.ll)
   emith("declare void @snd_play(ptr, i32, i32, i32)\n")
   emith("declare void @snd_stop(ptr)\n")