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-falloff.md

@@ -0,0 +1,31 @@
+---
+id: light-falloff
+name: Light.falloff
+category: light
+kind: namespace-method
+tokens: Light.falloff
+sig: Light.falloff(exponent)
+tip: Set the brightness-ramp exponent for later lights (1 linear, 2 quadratic…).
+order: 6
+ns: Light
+member: falloff
+---
+
+Sets the **falloff curve** applied to every light emitted after it. A light's brightness ramps from full at the centre to zero at its radius; the ramp is raised to <code>exponent</code>, so <code>1</code> is the original linear falloff, <code>2</code> is quadratic (a softer, more concentrated core), <code>3</code> cubic, and so on. Like the occluder store, the setting persists across frames until changed — a game sets it once, or per light, and it rides alongside the short <a href="light-point"><code>Light.point</code></a> / <a href="light-spot"><code>Light.spot</code></a> signatures. Integer exponent, Q16.16 ramp — deterministic.
+
+Parameters:
+- `exponent` — the ramp power, an integer `>= 1` (`1` = linear, the default)
+
+```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.falloff(2)                                        # a tighter, quadratic glow
+    Light.point(80, 60, 70, Color.rgb(255, 210, 130), 1.0)
+    Screen.show()
+  }
+}
+```