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>

docs/language/input/input-down.md

@@ -0,0 +1,24 @@
+---
+id: input-down
+name: Input.down
+category: input
+kind: namespace-method
+tokens: Input.down
+sig: Input.down(action: str) -> bool
+tip: Is a named action held on the frame last polled?
+order: 4
+ns: Input
+member: down
+---
+
+Returns whether a named action is <em>held</em> on the frame last read by <a href="input-poll.html"><code>Input.poll</code></a> — true when the polled key is one of the keys bound to the action. Use it for continuous input (move while held); for a one-shot press use <a href="input-pressed.html"><code>Input.pressed</code></a>. Reading an action rather than a key is what lets the same handler serve any binding.
+
+```ludic
+program Held {
+  entry {
+    Input.bind("up", 'w')
+    Input.poll()
+    if Input.down("up") { print(1) }
+  }
+}
+```