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>

docs/language/world/world-despawn.md

@@ -0,0 +1,28 @@
+---
+id: world-despawn
+name: World.despawn
+category: world
+kind: namespace-method
+tokens: World.despawn
+sig: World.despawn(entity)
+tip: Despawn an entity by id — runs its @OnDespawn hooks and frees the slot.
+order: 16
+ns: World
+member: despawn
+---
+
+Despawns an entity by id: it runs the entity's model <code>@OnDespawn</code> hook (with reason <code>Despawned</code>), sweeps any entity-scoped event listeners, and frees the slot back to the pool for reuse — the reflective, by-id form of the <code>despawn</code> statement. Use it from a system that has an entity id in hand (the engine world-bounds kill policy uses it, #84) or wherever you would write <code>despawn e</code> but only have the id.
+
+```ludic
+program Demo {
+  property Mob { hp: int = 0 }
+  model M { Mob }
+  entry {
+    spawn M { Mob { hp: 1 } }
+    let pm = World.prop_id("Mob")
+    let e = World.query_next(pm, 0)
+    World.despawn(e)
+    print(World.query_next(pm, 0))     # -1 — none left
+  }
+}
+```