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>

docs/language/input/input-cursor_mode.md

@@ -0,0 +1,34 @@
+---
+id: input-cursor_mode
+name: Input.cursor_mode
+category: input
+kind: namespace-method
+tokens: Input.cursor_mode
+sig: Input.cursor_mode(mode)
+tip: Hide / lock / confine the OS mouse cursor to the window (windowed).
+order: 36
+ns: Input
+member: cursor_mode
+---
+
+Sets how a windowed game captures the OS mouse cursor:
+
+- <code>0</code> normal (default) — the cursor is visible and free.
+- <code>1</code> hidden — the cursor is hidden while the window is focused, so a game can draw its own reticle.
+- <code>2</code> locked — hidden and dissociated from the mouse; the mouse feeds <em>relative</em> motion through <a href="input-mouse_dx.html"><code>Input.mouse_dx</code></a> / <code>mouse_dy</code>, and <a href="input-mouse_x.html"><code>Input.mouse_x</code></a> / <code>mouse_y</code> is a clamped virtual cursor — the FPS / twin-stick aim mode.
+- <code>3</code> confined — dissociated but visible; the mouse cannot leave the window.
+
+The platform **auto-releases** (shows the cursor and reconnects it) while the window is not the key window (Cmd-Tab), and restores it on window close, so the cursor is never left captured. On a headless / non-windowed build it is a no-op.
+
+```ludic
+program Aim {
+  property Tag { n: int = 0 }
+  model P { Tag }
+  @OnStart handler Boot { Input.cursor_mode(2) }     # lock + hide for aiming
+  handler Draw phase Render {
+    Screen.clear(0x101018)
+    Screen.fill_rectangle(Input.mouse_x() - 2, Input.mouse_y() - 2, 4, 4, 0xffcc00)
+    Screen.show()
+  }
+}
+```