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-ambient.md

@@ -0,0 +1,30 @@
+---
+id: light-ambient
+name: Light.ambient
+category: light
+kind: namespace-method
+tokens: Light.ambient
+sig: Light.ambient(color)
+tip: Multiply the whole scene by a tint — the night/cave modulate.
+order: 1
+ns: Light
+member: ambient
+---
+
+Multiplies every framebuffer pixel by <code>color</code> (a <code>0x00RRGGBB</code> tint), channel by channel (<code>channel * tint / 255</code>). This is the global modulate that sets mood before any light adds brightness back: a dim blue-grey gives night, a warm brown gives a cave, and <code>0xFFFFFF</code> is a no-op. Call it once, after drawing the scene and before the <a href="light-point"><code>Light.point</code></a> calls that light it back up.
+
+Parameters:
+- `color` — the ambient tint, `0x00RRGGBB` (darker/cooler = a darker scene)
+
+```ludic
+program Demo {
+  property Tag { v: int = 0 }
+  model Marker { Tag }
+  handler Render phase Update {
+    Screen.clear(Color.rgb(120, 120, 140))
+    Light.ambient(Color.rgb(48, 48, 72))   # dusk: dim and cool
+    Light.point(160, 120, 90, Color.Amber, 1.0)
+    Screen.show()
+  }
+}
+```