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>

docs/language/light/light-clear_occluders.md

@@ -0,0 +1,30 @@
+---
+id: light-clear_occluders
+name: Light.clear_occluders
+category: light
+kind: namespace-method
+tokens: Light.clear_occluders
+sig: Light.clear_occluders()
+tip: Forget every occluder — call once per frame before re-registering.
+order: 4
+ns: Light
+member: clear_occluders
+---
+
+Forgets every rectangle previously passed to <a href="light-occlude"><code>Light.occlude</code></a>. Occluders accumulate across calls and persist between frames, so a game that casts shadows calls this once at the top of its render phase and then re-registers the walls that should block light this frame — the same clear-then-rebuild rhythm as an immediate-mode draw list. With no occluders registered, <a href="light-point"><code>Light.point</code></a> lights its whole radius unobstructed.
+
+Parameters: none.
+
+```ludic
+program Demo {
+  property Tag { v: int = 0 }
+  model Marker { Tag }
+  handler Render phase Update {
+    Screen.clear(0)
+    Light.clear_occluders()                       # start the frame with no shadows
+    Light.occlude(90, 60, 6, 40)
+    Light.point(40, 80, 100, Color.rgb(255, 255, 255), 1.0)
+    Screen.show()
+  }
+}
+```