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-time_of_day.md

@@ -0,0 +1,30 @@
+---
+id: light-time_of_day
+name: Light.time_of_day
+category: light
+kind: namespace-method
+tokens: Light.time_of_day
+sig: Light.time_of_day(t)
+tip: Set the ambient tint from a 0..1 time-of-day — a day/night cycle in one value.
+order: 13
+ns: Light
+member: time_of_day
+---
+
+Sets the scene's <a href="light-ambient"><code>ambient</code></a> tint from a single **time-of-day** phase <code>t</code> — a <code>fixed</code> in <code>[0, 1]</code> where <code>0</code> is midnight, <code>0.25</code> dawn, <code>0.5</code> noon and <code>0.75</code> dusk. It ramps a deep-blue night up to full daylight and back over the day, so a game animates one value and the whole world's mood follows, without wiring the ambient colour by hand. It drives the same multiplicative modulate as <code>Light.ambient</code>, so call it once before adding lights. Deterministic (Q16.16 triangle ramp).
+
+Parameters:
+- `t` — the time of day, a `fixed` in `[0, 1]` (`0` = midnight, `0.5` = noon)
+
+```ludic
+program Demo {
+  property Clock { t: int = 0 }
+  model World { Clock }
+  handler Render phase Update {
+    Screen.clear(Color.rgb(255, 255, 255))
+    Light.time_of_day(0.5)                                 # high noon — full daylight
+    Light.point(80, 60, 60, Color.rgb(255, 240, 200), 1.0)
+    Screen.show()
+  }
+}
+```