feat(rendering): add Light.* — deterministic 2D light accumulation with hard shadows (#4)

A software light pass over the framebuffer, run in a render phase after drawing
the scene: Light.ambient multiplies the scene toward a tint (night/cave mood),
Light.point additively accumulates a radial glow with linear falloff clamped per
channel, and Light.occlude / Light.clear_occluders cast hard shadows by blocking
a light's rays against rectangular occluders. Integer + Q16.16 fixed throughout,
so a scene lights identically every run and in a headless render (diffable).

Engine in runtime/native/light.ludic, spliced on demand (g_uses_light) like the
regex/query runtimes; namespace wired in emit_call.ludic. Ships issue #4 tiers 1
(ambient + additive radial lights) and 2 (hard shadows). Normal-mapped sprites,
soft shadows, a day/night directional light, and auto-consuming Light2D/Occluder
components are follow-ups (the auto-system hook is tracked by #43).

- runtime/native/light.ludic: the light-accumulation engine (isqrt falloff,
  segment/occluder shadow test, ambient modulate)
- examples/library/lighting.ludic: 14 pixel-readback assertions
- docs/language/light/: Light.ambient/point/occlude/clear_occluders
- tools/x/test.ludic: lighting.ludic in the regression suite

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

selfhost/backend/emit_call.ludic

@@ -238,6 +238,16 @@ function emit_ns_call(ns: pointer, meth: pointer, e: Node) -> Val {
     if (meth == "flood")         { bare = "grid_flood";         push(labels, "x"); push(labels, "y"); push(labels, "wall") }
     if (meth == "a_star")        { bare = "path_a_star";        push(labels, "x0"); push(labels, "y0"); push(labels, "x1"); push(labels, "y1"); push(labels, "wall") }
   }
+  # Light.* — the 2D light-accumulation pass (runtime/native/light.ludic, spliced
+  # on demand). A game runs it in its render phase: ambient multiplies the scene
+  # down, point adds a radial glow (blocked by occluders -> hard shadows). Screen
+  # space, deterministic (integer + Q16.16), diffable. `energy` is a fixed.
+  if (ns == "Light") {
+    if (meth == "ambient")         { bare = "light_ambient";         push(labels, "color") }
+    if (meth == "point")           { bare = "light_point";           push(labels, "x"); push(labels, "y"); push(labels, "radius"); push(labels, "color"); push(labels, "energy") }
+    if (meth == "occlude")         { bare = "light_occlude";         push(labels, "x"); push(labels, "y"); push(labels, "width"); push(labels, "height") }
+    if (meth == "clear_occluders") { bare = "light_clear_occluders" }
+  }
   # Query.* — ECS spatial queries over the reflection ABI (runtime/native/query.ludic,
   # spliced on demand). `prop` is a property id (World.prop_id); the spatial forms
   # read two int fields (field ids) as (x, y). nearest/first return an entity (-1 =