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>

changes/light-tiers.md

@@ -0,0 +1,12 @@
+bump: minor
+type: feat
+**Lighting render-quality tiers (#49).** The 2D light pass gains the tiers 3-4
+from the original proposal, all on the same deterministic accumulation core:
+`Light.spot` cone/flashlight lights (direction + spread), a `Light.falloff`
+exponent (linear → quadratic → …), `Light.soft` soft shadows (occluder edges
+fade through a penumbra), `Light.gel` colour cookies (centre → rim tint),
+normal-mapped surfaces via `Light.normal` + `Light.height` (surfaces shade by
+facing, N·L), and a `Light.time_of_day` day/night ambient ramp. The engine
+lighting system consumes matching optional `Light2D` fields
+(`direction`/`spread`/`falloff`/`softness`/`gel`) too. Every tier is integer +
+Q16.16 fixed, so scenes light identically on every run and headless.