feat(input,ecs): #79 Input.axis_i directional int + #84 world bounds

#79 — Input.axis_i(neg,pos) -> int returns a -1/0/1 movement intent from the
multi-key device set, so WASD-to-movement needs no bool->int glue and feeds an
int mover directly (dx = Input.axis_i('a','d')).

#84 — a Bounds config entity (rect + policy, from ludic.core) drives the
engine-owned world-bounds system (LateUpdate): clamp / wrap / bounce (clamp +
flip Body velocity) / kill (despawn a body fully outside). Reads the Collider
size so the box stays inside; off by default, spliced only when Bounds is
declared (byte-identical otherwise). Adds World.despawn(e) — the by-id
reflective despawn (runs @OnDespawn + frees) via a new world_despawn intrinsic
whose @fn_world_despawn helper is emitted in emit_program's tail once a use is
seen (the g_uses_* prelude pattern), used by the kill policy and callable from
any system.

Examples input_movement + world_bounds. Full suite 112/0, goldens
byte-identical, fixpoint holds.

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

runtime/native/input.ludic

@@ -370,6 +370,17 @@ function input_axis(neg: int, pos: int) -> fixed {
   if in_bit_get(in_held, neg) { v = v - fixed(1) }
   return v
 }
+# #79 — a directional intent as a plain int: +1 if the positive key is held, -1 if
+# the negative, 0 if neither or both. Reads the multi-key device set, so it needs
+# no bool->int glue (the `dx = ki(key_down('d')) - ki(key_down('a'))` boilerplate)
+# and feeds an int mover (TopDown.move) straight: dx = Input.axis_i('a','d').
+function input_axis_i(neg: int, pos: int) -> int {
+  in_init()
+  var v = 0
+  if in_bit_get(in_held, pos) { v = v + 1 }
+  if in_bit_get(in_held, neg) { v = v - 1 }
+  return v
+}
 # A 2D vector from four direction keys, normalized so a diagonal is not faster.
 function input_vector(left: int, right: int, up: int, down: int) -> Vector {
   in_init()