feat(input): #83 Input Manager + auto-commit the device layer in the frame loop

Fixes the papercut: the generated frame loop called rt_poll() (feeding only
Input.key) but never input_poll(), so Input.active/key_down/mouse/pad read
empty unless the game called Input.poll() by hand. The loop now calls
input_poll() when the game uses any Input runtime method — committing the
held-key/mouse/gamepad state, and record/replay — and stores its return as the
frame key so Input.key still works. A game using no Input runtime keeps the
plain rt_poll path, byte-identical.

Adds the Input-Manager API: Input.action(name,key) ships a default binding
(kept if already bound, so a rebind/loaded map isn't clobbered),
Input.bind_pad(name,button) makes an action device-agnostic (keyboard OR pad),
and Input.active/just_pressed/just_released read the multi-key device layer
with clean on-press/on-release edges (deterministic, dispatch-free — a handler
polls the edge; a replay fires identically).

Examples input_manager + input_auto, 5 docs pages. Full suite 107/0, goldens
byte-identical, fixpoint holds.

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

selfhost/backend/game/emit_world.ludic

@@ -467,9 +467,20 @@ function emit_game_main() -> void {
     emit("  br i1 ");  emit(rc);  emit(", label %body, label %done\n")
   }
   emit("body:\n")
-  if (find_fn("rt_poll") != null) {
-    let k = emit_bind("call i32 @fn_rt_poll()")
+  # #83: when the game uses the input action-map / device layer, the frame loop
+  # commits the device layer automatically each frame by calling input_poll (which
+  # reads the live key via rt_poll, records/replays, and rebuilds the held-key set,
+  # mouse and gamepad state). Its return is the frame key, so Input.key still works
+  # and the game no longer has to call Input.poll by hand. A game that uses no Input
+  # runtime keeps the plain rt_poll path, byte-identical.
+  if g_uses_input and (find_fn("input_poll") != null) {
+    let k = emit_bind("call i32 @fn_input_poll()")
     emit("  store i32 ");  emit(k);  emit(", ptr @L_key\n")
+  } else {
+    if (find_fn("rt_poll") != null) {
+      let k = emit_bind("call i32 @fn_rt_poll()")
+      emit("  store i32 ");  emit(k);  emit(", ptr @L_key\n")
+    }
   }
   emit_calls_for_phase("Input")
   emit_calls_for_phase("FixedUpdate")