feat(light): render-quality tiers 3-4 — cones, falloff, soft shadows, gels, normals, day/night (#49)

The remaining lighting tiers from the original proposal, all extending the
deterministic accumulation core (light.ludic) — no new ECS plumbing:

- Light.spot: cone / flashlight lights (direction + spread degrees), with a
  self-contained integer atan2-in-degrees and a feathered edge.
- Light.falloff: a brightness-ramp exponent (1 linear, 2 quadratic, …) via
  repeated fixed multiply.
- Light.soft: soft shadows — an area-sampled light so an occluder edge fades
  through a penumbra instead of a hard cut.
- Light.gel + Light.clear_gel: colour cookies — a light gels from its centre
  colour to a rim colour.
- Light.normal + Light.clear_normals + Light.height: a normal G-buffer so
  surfaces shade by facing (N·L), not distance alone (tier 3).
- Light.time_of_day: a day/night ambient ramp from a single 0..1 value.

The engine lighting system (systems_light.ludic) consumes matching optional
Light2D fields — direction/spread/falloff/softness/gel — each defaulting off so
an older five-field Light2D lights exactly as before. Every tier is integer +
Q16.16 fixed, so scenes light identically on every run and headless.

Worked example + regression: examples/library/light_tiers.ludic (1 1 1 1 1 1 1 1 1).
Nine new docs/language/light pages. Full suite 76 passed, self-host C-free
fixpoint intact, no golden drift.

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

docs/language/light/light-gel.md

@@ -0,0 +1,32 @@
+---
+id: light-gel
+name: Light.gel
+category: light
+kind: namespace-method
+tokens: Light.gel
+sig: Light.gel(color)
+tip: A colour cookie — later lights gel from their centre colour to this rim colour.
+order: 8
+ns: Light
+member: gel
+---
+
+Sets a **gel** (a colour cookie) for lights emitted after it: the light blends from its own <code>color</code> at the centre to this <code>color</code> at the rim, so a single light can carry a two-tone tint — a warm core cooling to a blue edge, a fire that reddens outward. Clear it with <a href="light-clear_gel"><code>Light.clear_gel</code></a> to return to a flat single-colour light. The gel rides on both <a href="light-point"><code>Light.point</code></a> and <a href="light-spot"><code>Light.spot</code></a>, and persists until cleared. Per-channel Q16.16 interpolation — deterministic.
+
+Parameters:
+- `color` — the rim colour, `0x00RRGGBB`
+
+```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(24, 24, 32))
+    Light.gel(Color.rgb(40, 60, 160))                      # cool blue rim
+    Light.point(80, 60, 70, Color.rgb(255, 200, 120), 1.0) # warm core
+    Light.clear_gel()
+    Screen.show()
+  }
+}
+```