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-occlude.md

@@ -0,0 +1,32 @@
+---
+id: light-occlude
+name: Light.occlude
+category: light
+kind: namespace-method
+tokens: Light.occlude
+sig: Light.occlude(x, y, width, height)
+tip: Register a rectangle that blocks light — a hard shadow caster.
+order: 3
+ns: Light
+member: occlude
+---
+
+Registers an axis-aligned rectangle (screen space) that blocks light: for every <a href="light-point"><code>Light.point</code></a> that follows, any pixel whose ray from the light centre crosses this rectangle — or lies inside it — is left in shadow, carving a hard umbra behind the wall. Register the geometry that should cast shadows this frame, then draw the lights. Occluders persist until <a href="light-clear_occluders"><code>Light.clear_occluders</code></a>, so call that once per frame first; up to 64 occluders are kept.
+
+Parameters:
+- `x`, `y` — the top-left corner, in screen pixels
+- `width`, `height` — the rectangle size in pixels
+
+```ludic
+program Demo {
+  property Wall { x: int = 0, y: int = 0 }
+  model Block { Wall }
+  handler Render phase Update {
+    Screen.clear(0)
+    Light.clear_occluders()
+    Light.occlude(120, 80, 8, 48)                 # a pillar
+    Light.point(60, 100, 120, Color.rgb(255, 240, 200), 1.0)
+    Screen.show()
+  }
+}
+```