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/_section.md

@@ -4,4 +4,4 @@ title: Light
 order: 33
 ---
 
-A software 2D light-accumulation pass over the framebuffer, run in a render phase after drawing the scene and before <a href="screen-show"><code>Screen.show</code></a>. <a href="light-ambient"><code>Light.ambient</code></a> multiplies the whole scene toward a tint — the night/cave modulate that darkens everything so lights add mood back on top. <a href="light-point"><code>Light.point</code></a> accumulates a radial glow that falls off with distance and clamps per channel, and <a href="light-occlude"><code>Light.occlude</code></a> registers rectangles that block a light's rays to cast hard shadows (cleared each frame with <a href="light-clear_occluders"><code>Light.clear_occluders</code></a>). Lighting is a rendering concern only — it never touches game state or replays — and it is fully deterministic (integer and Q16.16 fixed), so the same scene lights identically every run and in a headless render, keeping screenshots diffable. Colours are <code>0x00RRGGBB</code>. This is the imperative surface; consuming <code>Light2D</code>/<code>Occluder</code> components automatically, normal-mapped sprites and soft shadows are planned follow-ups. Related: <a href="screen"><code>Screen</code></a>, <a href="color"><code>Color</code></a>, <a href="camera"><code>Camera</code></a>.
+A software 2D light-accumulation pass over the framebuffer, run in a render phase after drawing the scene and before <a href="screen-show"><code>Screen.show</code></a>. <a href="light-ambient"><code>Light.ambient</code></a> multiplies the whole scene toward a tint — the night/cave modulate that darkens everything so lights add mood back on top. <a href="light-point"><code>Light.point</code></a> accumulates a radial glow that falls off with distance and clamps per channel, and <a href="light-occlude"><code>Light.occlude</code></a> registers rectangles that block a light's rays to cast hard shadows (cleared each frame with <a href="light-clear_occluders"><code>Light.clear_occluders</code></a>). Lighting is a rendering concern only — it never touches game state or replays — and it is fully deterministic (integer and Q16.16 fixed), so the same scene lights identically every run and in a headless render, keeping screenshots diffable. Colours are <code>0x00RRGGBB</code>. Beyond the radial core, the pass carries the render-quality tiers: <a href="light-spot"><code>Light.spot</code></a> cones, a <a href="light-falloff"><code>Light.falloff</code></a> exponent, <a href="light-soft"><code>Light.soft</code></a> shadows (penumbra), <a href="light-gel"><code>Light.gel</code></a> colour cookies, normal-mapped surfaces (<a href="light-normal"><code>Light.normal</code></a> + <a href="light-height"><code>Light.height</code></a>) that shade by facing, and a <a href="light-time_of_day"><code>Light.time_of_day</code></a> day/night ramp — every one deterministic. This is the imperative surface; the engine also consumes <code>Light2D</code>/<code>Occluder</code> components automatically (a Light2D may carry optional <code>direction</code>/<code>spread</code>/<code>falloff</code>/<code>softness</code>/<code>gel</code> fields). Related: <a href="screen"><code>Screen</code></a>, <a href="color"><code>Color</code></a>, <a href="camera"><code>Camera</code></a>.