fix(world): reallocate shadow map after Suspense + clear LaFabrik doorway

Shadows occasionally failed to render on initial load and the Fabrik
doorway sometimes blocked the player. Both issues are tracked down to
geometry that mounts after Lighting:

- Shadows: GLTFs and the merged static map mount imperatively after
  Lighting, so materials get compiled against a renderer state that
  pre-dates the final scene and bake a 'no shadow map' permutation,
  silently dropping shadows. A WebGL context-restore cycle fixes it,
  but is too invasive. New 'useShadowMapWarmup' hook replays it
  cheaply: once the scene mesh count has been stable for ~1s, it
  disposes the directional shadow map (three.js reallocates it on
  the next render) and marks every material 'needsUpdate' so shaders
  rebind to the freshly created shadow sampler.
- Doorway: the door slab + its Solidify-modifier frame (children of
  the 'Thicken' parent in the LaFabrik GLTF) sat inside the doorway
  AABB and prevented the player from walking through. Stripped from
  the collision octree alongside the existing 'porte' slab; visual
  rendering is unaffected.

Also: extract sun-relative-to-camera placement into a small helper,
remove the temporary diagnostic logs, and document the shadow warmup
in three-debugging.md.

docs/technical/three-debugging.md

@@ -44,30 +44,39 @@ through opaque geometry.
 ## Shadow rendering intermittence
 
 Shadows occasionally failed to render on initial load and could disappear
-mid-session even though the `Lighting` configuration ran to completion.
+mid-session even though the `Lighting` configuration ran to completion. The
+fix has two layers:
 
-Root cause: the sun follows the camera (its world matrix is dirty every frame
-via `updateMatrixWorld()` inside `Lighting.useFrame`). With `shadow.autoUpdate`
-alone, three.js can skip the shadow map re-render on a frame where the matrix
-update has happened but the renderer's internal dirty tracking does not pick
-it up, leaving the shadow map stale or unrendered.
+### Per-frame refresh (steady state)
 
-Fix in `src/world/Lighting.tsx`: explicit `sun.shadow.needsUpdate = true` in
-two places, restoring the belt-and-suspenders pattern from `develop`:
+The sun follows the camera, so its world matrix is dirty every frame. With
+`shadow.autoUpdate` alone, three.js can skip the shadow map re-render on a
+frame where the matrix update has happened but the renderer's internal dirty
+tracking does not pick it up. To prevent that, `Lighting.useFrame` sets
+`sun.shadow.needsUpdate = true` after the per-frame matrix updates. Shadow
+config is centralized in `src/data/world/lightingConfig.ts` (`bias=0`,
+`normalBias=0`, `cameraSize=95`).
 
-- After `configureSunShadow(...)` in the mount `useEffect`.
-- At the end of the `useFrame` block, right after `sun.updateMatrixWorld()`.
+### Mount-time shadow map reallocation (`useShadowMapWarmup`)
 
-Mitigations also in place:
+The merged static map and other GLTFs mount imperatively after `Lighting`,
+so the shadow render target ends up linked to a renderer state that pre-dates
+the final scene. Materials compiled at that point bake a "no shadow map"
+permutation into their shader program and silently fail to render shadows
+until a WebGL context-restore cycle (the kind triggered by Chrome DevTools
+in `?debug` runs) reallocates everything.
 
-- Shadow config centralized in `src/data/world/lightingConfig.ts`
-  (`bias=0`, `normalBias=0`, `cameraSize=95`).
-- Late-suspension Suspense boundaries in `World.tsx` to prevent global scene
-  remounts that would re-run shadow setup mid-load.
-- `gl.shadowMap.needsUpdate = true` on `onCreated` and on
-  `webglcontextrestored` in `src/pages/page.tsx`.
+`src/hooks/three/useShadowMapWarmup.ts` replays that cycle programmatically
+without the cost of a full context loss. It runs a `useFrame` watchdog that
+samples the scene mesh count every 6 frames; once the count has been stable
+for ~1 s (or after a 5 s safety cap), it:
 
-If the issue reproduces, capture `[diag]`-style logs from `useOctreeGraphNode`,
-`Lighting`, and `GameMapCollision` to confirm there is no extra configuration
-pass (which would indicate a remaining suspending hook outside the existing
-Suspense boundaries).
+1. Disposes the directional light shadow map and nulls it. three.js
+   reallocates the render target on the next render at the configured
+   `mapSize`.
+2. Marks every material's `needsUpdate = true`, forcing a shader recompile
+   that rebinds every program to the freshly created shadow sampler.
+3. Forces a single shadow pass and invalidates the renderer.
+
+The watchdog runs once per mount and adds a single traversal every 6 frames
+during the warmup window, after which it self-terminates.