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>

examples/library/input_actions.ludic

@@ -0,0 +1,37 @@
+# input_actions.ludic — action maps, runtime rebinding, and deterministic input
+# record/replay (#7). Gameplay reads named actions, not physical keys, so a key
+# is rebindable; and because the sim is deterministic in its input stream, a
+# recorded run replays exactly. Driven headless off stdin (one key per poll):
+#
+#   printf '  xwa' | bin/ludic examples/library/input_actions.ludic   ->  1 0 1 1 0 1 0
+program InputActions {
+  function bi(b: bool) -> int { if b { return 1 };  return 0 }
+
+  entry {
+    Input.bind("jump", ' ')          # space triggers "jump"
+    Input.bind("up", 'w')            # w triggers "up"
+
+    # --- action map + runtime rebinding (live) ---
+    Input.poll()                     # stdin: ' '
+    print(bi(Input.down("jump")))    # 1 — space is bound to jump
+
+    Input.rebind("jump", ' ', 'x')   # remap jump from space to x at runtime
+    Input.poll()                     # stdin: ' '
+    print(bi(Input.down("jump")))    # 0 — space no longer fires jump
+    Input.poll()                     # stdin: 'x'
+    print(bi(Input.down("jump")))    # 1 — x now fires jump
+
+    # --- deterministic record / replay ---
+    Input.record()                   # capture the polled keys from here
+    Input.poll()                     # stdin: 'w'
+    print(bi(Input.down("up")))      # 1
+    Input.poll()                     # stdin: 'a'
+    print(bi(Input.down("up")))      # 0
+
+    Input.replay()                   # rewind; poll now reads the tape, not stdin
+    Input.poll()
+    print(bi(Input.down("up")))      # 1 — reproduces the recorded frame exactly
+    Input.poll()
+    print(bi(Input.down("up")))      # 0
+  }
+}