docs(world): document map lod system

docs/technical/map-lod.md

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+# Map LOD System
+
+This document describes the runtime LOD system used by the production map.
+
+## Goal
+
+The map now supports two visual versions for selected models:
+
+- the regular model in `public/models/<name>/`
+- the lighter model in `public/models/<name>-LOD/`
+
+The runtime chooses between those paths from the active graphics preset. This keeps nearby objects visually richer while reducing the cost of distant objects.
+
+## Graphics Presets
+
+Presets are configured in:
+
+```txt
+src/data/world/graphicsConfig.ts
+```
+
+Current behavior:
+
+| Preset   | Chunk load distance | Fog | LOD behavior                          |
+| -------- | ------------------: | --- | ------------------------------------- |
+| `low`    |                 10m | On  | Always use `*-LOD` models             |
+| `medium` |                 20m | On  | Always use `*-LOD` models             |
+| `high`   | Current default 50m | Off | Regular model up to 10m, then `*-LOD` |
+| `ultra`  |                 50m | Off | Regular model up to 20m, then `*-LOD` |
+
+The unload distance stays slightly larger than the load distance to avoid rapid mount/unmount flickering when the player stands near a boundary.
+
+## Runtime Selection
+
+LOD path mapping lives in:
+
+```txt
+src/data/world/mapLodConfig.ts
+```
+
+The main selector is `selectMapModelPathByDistance()`. It receives:
+
+- the current camera distance
+- the map model name
+- the regular model path
+- the active graphics preset
+
+It returns either the regular path or the `*-LOD` path.
+
+## Chunked Instanced Models
+
+Repeated static assets are rendered through:
+
+```txt
+src/world/map-instancing/MapInstancingSystem.tsx
+```
+
+For each visible chunk, the system checks the nearest instance in that chunk. If the nearest instance is inside the high-detail threshold, the whole chunk uses the regular model. Otherwise, it uses the `*-LOD` model.
+
+This is intentionally chunk-level LOD instead of per-instance LOD. It matches the existing chunk streaming architecture and avoids splitting every object into many tiny batches.
+
+## Single And Generated Models
+
+Single map nodes use:
+
+```txt
+src/hooks/world/useMapLodModelPath.ts
+src/world/GameMap.tsx
+```
+
+Some named map objects are rendered through dedicated generated components instead of the generic `GameMap` path. Those components must call `useMapLodModelPath()` directly.
+
+Current dedicated generated components with LOD support:
+
+```txt
+src/components/three/world/EcoleModel.tsx
+src/components/three/world/LaFabrikMapModel.tsx
+```
+
+This matters for `lafabrik`: adding `public/models/lafabrik-LOD/` is not enough by itself. The component must also be connected to `useMapLodModelPath()`.
+
+## Adding A New LOD Model
+
+To add LOD support for a model:
+
+1. Add the light model in `public/models/<name>-LOD/model.gltf`.
+2. Keep the regular model in `public/models/<name>/model.gltf`.
+3. Add the mapping in `src/data/world/mapLodConfig.ts`.
+4. If the model uses a dedicated component, call `useMapLodModelPath()` in that component.
+5. Preload both paths when the component is dedicated and uses `useGLTF.preload()`.
+6. Verify the GLTF references: buffers, textures, opacity maps, and relative paths.
+
+## Current LOD Models
+
+The current explicit LOD mappings are:
+
+```txt
+ebike
+eolienne
+pylone
+boiteimmeuble
+ecole
+immeuble1
+lafabrik
+maison1
+panneauaffichage
+talkie
+```
+
+## Regression Risks
+
+The most common failure modes are:
+
+- the `*-LOD` folder exists but is missing from `mapLodConfig.ts`
+- a dedicated generated component keeps a hardcoded model path
+- GLTF references point to textures that were renamed during export
+- a model is added to LOD config but does not spawn through `GameMap` or `MapInstancingSystem`
+
+Before committing model changes, validate both the regular and LOD folders for missing GLTF refs.