feat(ecs): Light2D/Occluder/Ambient as auto-consumed components (#47)

The ECS-native shape the 2D lighting follow-up asked for, built on the
engine-owned-system hook from #43. A torch is just an entity carrying
Light2D, a wall an entity carrying Occluder, and one Ambient entity sets
the night tint — the engine runs the whole deterministic light pass at the
end of the Render phase (ambient modulate -> carve occluder shadows ->
accumulate additive radial lights) and presents. No Light.* calls wired.

- runtime/native/systems_light.ludic: esys_light2d, consuming the components
  through the by-name reflection ABI and reusing the #4 accumulation core
  (light_ambient / light_occlude / light_point). Reads position from a
  Position component when present, else the light's own x/y.
- Split from systems.ludic so a SpriteAnim/Motion-only game never links the
  light pass; spliced (with light.ludic) only when Light2D/Occluder declared.
- emit_main now boots rt_init like the auto-loop/test runner, so an
  entry-driven game that renders has its framebuffer allocated (headless:
  allocate only, no window, byte-identical stdout for non-rendering games).

Worked example + regression: examples/library/light_ecs.ludic (32 1 32 1).
Full suite 74 passed, self-host C-free fixpoint intact, no golden drift.

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

LANGUAGE.md

@@ -270,6 +270,9 @@ enough, and a game that declares none is byte-for-byte unchanged.
 |---|---|---|
 | `SpriteAnim { ticks, fps, frames, mode, frame }` | `Update` | advances `frame` — spritesheet frame animation (`mode` 0 loop, 1 once, 2 ping-pong) |
 | `Motion { ticks, dur, from, to, ease, value, done }` | `Update` | advances `value` — value tween (`ease` 0 linear, 1 in, 2 out, 3 in-out), latches `done` |
+| `Light2D { x, y, radius, color, intensity }` | `Render` | additive radial glow; the engine runs the whole 2D light pass and presents |
+| `Occluder { x, y, w, h }` | `Render` | a rectangular shadow caster the light pass carves out |
+| `Ambient { color }` | `Render` | one entity tints the whole scene (night/cave) before lights accumulate |
 
 ```ludic
 # doc-check: skip — illustrative engine-owned system
@@ -279,9 +282,15 @@ model Hero { Pos, SpriteAnim }
 spawn Hero { Pos { x: 0, y: 0 }  SpriteAnim { fps: 10, frames: 6, mode: 0 } }
 ```
 
+A `Light2D` / `Occluder` reads its position from a `Position { x, y }` component
+on the same entity when the entity carries one, else from its own `x` / `y`
+fields — so "Position + Light2D" and a self-positioned light both work. With
+`Light2D` present the engine owns the frame flip: a draw handler renders the
+scene and does **not** call `Screen.show`.
+
 Everything is integer and deterministic (the frame clock ticks at a fixed 60/s),
-so animation and motion reproduce exactly under replay and lockstep netcode. See
-`examples/library/anim_ecs.ludic`.
+so animation, motion and lighting reproduce exactly under replay and lockstep
+netcode. See `examples/library/anim_ecs.ludic` and `examples/library/light_ecs.ludic`.
 
 ## Annotations