Add 2D lightmap: occlusion shadows and point lights

Extend the Lighting module with a per-cell lightmap. LightmapBuilder (pure,
tested) fills an ambient base, shades occluder cells, and adds point lights
that attenuate with distance and are blocked by occluders between source and
cell (grid-traced soft shadows). Lightmap holds the grid plus a greyscale
texture (Upload) and a bilinear SampleAt for the simulation; a PointLight
component places lights in the world. LightmapSystem rebuilds the grid a few
times a second (ambient from the day/night cycle with a night floor, occluders
from a game-supplied grid, point lights from the ECS); LightmapRenderSystem
multiplies the lightmap over the world after the sprite flush and under the
HUD. Wired via scene.UseLighting(...), sampled through Lighting.SampleAt. Docs
updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CLAUDE.md

@@ -38,8 +38,10 @@ Engine libraries (each feature is a namespaced subfolder of its host):
   (`MrGameEng.Tilemaps`: code-built tile grids rendered through the batcher,
   `scene.UseTilemaps()` after `UseRenderer2D()`, in `Graphics/Tilemaps/`) and **Lighting**
   (`MrGameEng.Lighting`, `Graphics/Lighting/`: day/night ambient over the `Calendar` driving
-  `Renderer2D.AmbientLight` on world layers, `scene.UseDayNight(renderer)`; a sampleable
-  `DayNight.SampleAt` for the simulation — point lights and shadows land here later). → `Core`.
+  `Renderer2D.AmbientLight` on world layers, `scene.UseDayNight(renderer)`; plus a per-cell
+  **lightmap** — `LightmapBuilder` (ambient × occlusion + point lights with grid-traced shadows),
+  `PointLight` component, multiplied over the world via `scene.UseLighting(...)`, sampleable with
+  `Lighting.SampleAt` for the simulation). → `Core`.
 - **`Audio`** — ogg playback (NVorbis); `AudioManager` with `SoundVolume`/`MasterVolume`
   (one knob for effects + music). → `Core`.
 - **`Content`** — the asset/content pipeline: **Assets** (`MrGameEng.Assets`: runtime