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-point.md

@@ -0,0 +1,33 @@
+---
+id: light-point
+name: Light.point
+category: light
+kind: namespace-method
+tokens: Light.point
+sig: Light.point(x, y, radius, color, energy)
+tip: Add a radial glow that falls off with distance.
+order: 2
+ns: Light
+member: point
+---
+
+Additively accumulates a radial point light centred at <code>(x, y)</code>. Brightness is full at the centre and falls off linearly to zero at <code>radius</code> pixels, scaled by <code>energy</code> (a <code>fixed</code> multiplier — <code>1.0</code> is full, <code>2.0</code> over-drives toward white, <code>0.5</code> is dim), and clamped per channel at 255 so overlapping lights add without wrapping. Only the light's bounding box is touched, and any <a href="light-occlude"><code>Light.occlude</code></a> rectangle between the centre and a pixel puts that pixel in shadow. Colours are <code>0x00RRGGBB</code>.
+
+Parameters:
+- `x`, `y` — the light centre, in screen pixels
+- `radius` — the reach in pixels; brightness is zero at and beyond it
+- `color` — the light colour, `0x00RRGGBB`
+- `energy` — a `fixed` brightness multiplier (`1.0` = full)
+
+```ludic
+program Demo {
+  property Torch { x: int = 0, y: int = 0 }
+  model Lamp { Torch }
+  handler Render phase Update {
+    Screen.clear(0)
+    Light.ambient(Color.rgb(32, 32, 48))
+    Light.point(80, 60, 70, Color.rgb(255, 210, 130), 1.0)   # a warm torch
+    Screen.show()
+  }
+}
+```