feat(input): raw device layer — multi-key held state, analog, mouse, gamepad, touch, full-state replay (#50)

The raw device layer the input proposal sketched, over the action maps +
record/replay of #7. Beyond one key per frame, gameplay can read:

- Multiple simultaneous held keys: Input.key_down / key_pressed / key_released,
  with clean rising/falling edges (hold left AND jump).
- Analog from keys: Input.axis(neg, pos) and a normalized Input.vector(l,r,u,d)
  (diagonals scaled by 1/sqrt(2)), plus Input.strength(action).
- Mouse: Input.mouse_x/y, mouse_dx/dy (per-frame delta), mouse_down(btn), wheel.
- Gamepads: Input.pad_connected/pad_button/pad_axis (SDL-order buttons, -1..1
  sticks); touch: Input.touch_count/touch_x/touch_y.

The held set is fed by the platform when windowed — cocoa.ll now tracks
keyDown/keyUp into a 256-bit held-key bitset (win_held) and the mouse
buttons/wheel (win_mouse), gated so headless builds DCE the native calls — and
by the Input.press / Input.set_mouse / Input.set_pad / Input.set_touch injection
on every target (Godot-style action injection: replays, AI, network-fed input).
Input.record / replay now snapshot the full per-frame device state (held set +
mouse), extending #7's single-key tape.

Everything is integer and deterministic, so the same inputs reproduce the same
frame on every run and headless. The gamepad/touch native hardware bindings
(GameController.framework / NSTouch) feed the same injected state and are the one
remaining platform-glue follow-up; the software layer, semantics and replay are
complete and driven deterministically today.

Worked example + regression: examples/library/input_device.ludic
(1 1 0 1 0 1 71 -71 5 1 3 1 2 1 0 0 1, injection-driven headless). 23 new
docs/language/input pages. Full suite 78 passed, self-host C-free fixpoint
intact, no golden drift; cocoa.ll assembles and a windowed build links.

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

docs/language/input/input-mouse_down.md

@@ -0,0 +1,24 @@
+---
+id: input-mouse_down
+name: Input.mouse_down
+category: input
+kind: namespace-method
+tokens: Input.mouse_down
+sig: Input.mouse_down(button) -> bool
+tip: Is a mouse button held?
+order: 20
+ns: Input
+member: mouse_down
+---
+
+Returns whether a mouse button is held this frame — button <code>0</code> left, <code>1</code> right, <code>2</code> middle/other.
+
+```ludic
+program Demo {
+  entry {
+    Input.set_mouse(0, 0, 1, 0)
+    Input.poll()
+    if Input.mouse_down(0) { print(1) }
+  }
+}
+```