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>

LANGUAGE.md

@@ -270,7 +270,7 @@ enough, and a game that declares none is byte-for-byte unchanged.
 |---|---|---|
 | `SpriteAnim { ticks, fps, frames, mode, frame }` | `Update` | advances `frame` — spritesheet frame animation (`mode` 0 loop, 1 once, 2 ping-pong) |
 | `Motion { ticks, dur, from, to, ease, value, done }` | `Update` | advances `value` — value tween (`ease` 0 linear, 1 in, 2 out, 3 in-out), latches `done` |
-| `Light2D { x, y, radius, color, intensity }` | `Render` | additive radial glow; the engine runs the whole 2D light pass and presents |
+| `Light2D { x, y, radius, color, intensity }` | `Render` | additive radial glow; the engine runs the whole 2D light pass and presents. Optional `direction`/`spread` (cone), `falloff`, `softness`, `gel` fields select the render-quality tiers |
 | `Occluder { x, y, w, h }` | `Render` | a rectangular shadow caster the light pass carves out |
 | `Ambient { color }` | `Render` | one entity tints the whole scene (night/cave) before lights accumulate |
 
@@ -286,7 +286,11 @@ A `Light2D` / `Occluder` reads its position from a `Position { x, y }` component
 on the same entity when the entity carries one, else from its own `x` / `y`
 fields — so "Position + Light2D" and a self-positioned light both work. With
 `Light2D` present the engine owns the frame flip: a draw handler renders the
-scene and does **not** call `Screen.show`.
+scene and does **not** call `Screen.show`. Beyond the radial core the light pass
+carries the render-quality tiers — `Light.spot` cones, a `Light.falloff`
+exponent, `Light.soft` shadows (penumbra), `Light.gel` colour cookies,
+normal-mapped surfaces (`Light.normal` + `Light.height`), and a
+`Light.time_of_day` day/night ramp — every one deterministic.
 
 ### Input actions & deterministic replay