feat(input): action maps + deterministic record/replay (#7)

The two ideas the input revamp leads with, built in Ludic over the
single-key poll every target already provides:

- Action maps: gameplay reads named actions, not physical keys, so keys
  are rebindable and a scheme is data. Input.bind(action, key),
  Input.down/pressed(action), Input.rebind(action, from, to).
- Deterministic record/replay: Input.poll() is the one per-frame input
  read; Input.record() captures the key each frame and Input.replay()
  feeds the tape back, so a run reproduces exactly — the seed of lockstep
  netcode. "Read input" and "read a recorded snapshot" are the same call.

runtime/native/input.ludic (spliced when the new Input.* methods are used;
pulls in core.ludic for rt_poll). emit_ns_call routes the methods to the
@fn_input_* runtime; parse.ludic gates the splice. Seven docs/language
pages; worked example + regression examples/library/input_actions.ludic
(1 0 1 1 0 1 0). Full suite 75 passed, self-host fixpoint intact, no golden
drift. The device layer (multi-key held, gamepads, touch, analog) needs a
platform key-state backend and is tracked separately.

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

LANGUAGE.md

@@ -288,6 +288,18 @@ fields — so "Position + Light2D" and a self-positioned light both work. With
 `Light2D` present the engine owns the frame flip: a draw handler renders the
 scene and does **not** call `Screen.show`.
 
+### Input actions & deterministic replay
+
+Beyond the raw `Input.key()` (this frame's key code), gameplay can read **named
+actions** instead of physical keys, so a key is rebindable and a control scheme
+is data. `Input.bind(action, key)` binds a key; `Input.down(action)` /
+`Input.pressed(action)` read it (held vs one-shot edge); `Input.rebind(action,
+from, to)` remaps it at runtime. `Input.poll()` is the single per-frame input
+read the actions sit on — which is what makes **deterministic replay** fall out:
+`Input.record()` captures the polled key each frame and `Input.replay()` feeds
+the tape back, so a run reproduces exactly (the seed of lockstep netcode). All
+integer and deterministic. See `examples/library/input_actions.ludic`.
+
 Everything is integer and deterministic (the frame clock ticks at a fixed 60/s),
 so animation, motion and lighting reproduce exactly under replay and lockstep
 netcode. See `examples/library/anim_ecs.ludic` and `examples/library/light_ecs.ludic`.