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-clear_gel.md

@@ -0,0 +1,30 @@
+---
+id: light-clear_gel
+name: Light.clear_gel
+category: light
+kind: namespace-method
+tokens: Light.clear_gel
+sig: Light.clear_gel()
+tip: Clear the gel — later lights are a flat single colour again.
+order: 9
+ns: Light
+member: clear_gel
+---
+
+Clears any <a href="light-gel"><code>Light.gel</code></a> set earlier, so lights emitted after it are a flat single colour again (no centre-to-rim tint). Pair it with <code>Light.gel</code> to scope a colour cookie to just one light.
+
+```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))
+    Light.point(60, 60, 60, Color.rgb(255, 200, 120), 1.0) # gelled
+    Light.clear_gel()
+    Light.point(150, 60, 60, Color.rgb(255, 240, 200), 1.0) # flat again
+    Screen.show()
+  }
+}
+```