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>

changes/input-axis-i.md

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+bump: minor
+type: feat
+Directional int input (#79) — `Input.axis_i(neg, pos) -> int` returns a -1/0/1 movement intent (`+1` positive key held, `-1` negative, `0` neither or both) read from the multi-key device set, so turning WASD into movement no longer needs the `ki(key_down('d')) - ki(key_down('a'))` bool-to-int glue and feeds an int mover directly: `dx = Input.axis_i('a', 'd')`, `dy = Input.axis_i('w', 's')`. Complements `Input.axis` (fixed) / `Input.vector` (normalized). Example: `examples/library/input_movement.ludic`.

changes/world-bounds.md

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+bump: minor
+type: feat
+Optional world boundaries (#84) — declare a single `Bounds` config entity (a rect `x, y, w, h` plus a policy, shipped from **ludic.core**) and the engine-owned world-bounds system keeps every moving `Body` inside the play area each frame, so a game no longer hand-clamps `Position`. Four policies: `0` clamp (walls), `1` wrap (toroidal), `2` bounce (clamp + flip the Body velocity on the axis that hit), `3` kill (despawn a body fully outside). It reads each body's `Collider` size so the whole box stays inside; off by default (no `Bounds` entity = open world). Registered on the engine-system registry for LateUpdate (after movement integrates) and spliced only when a game declares `Bounds`, so a game that never does compiles byte-identically. Also adds `World.despawn(entity)` — the reflective, by-id form of the `despawn` statement (runs `@OnDespawn` + frees the slot), which the kill policy uses and any system can call. Example: `examples/library/world_bounds.ludic`.