Reorganize docs into category folders and drop the retired head-review page

Group the top-level docs into docs/project, docs/design, docs/assets and
docs/process, each with its own README index. Delete the superseded
docs/screens/head-review page and sheets. Rewrite every markdown link,
absolute docs path and tool/test reference to the new locations, fix the
pre-existing broken anchors and remove the dead Joke Shop photo-board link.
check_docs OK, Python tooling 167/167.

docs/adr/0015-npc-model-variety-sourcing.md

@@ -21,7 +21,7 @@ A review compared free/open-source sources against that contract. Quaternius
 Ultimate Modular (CC0), Kenney Blocky Characters (CC0), Khronos CesiumMan and
 RiggedFigure (CC BY 4.0) and the MakeHuman base mesh (CC0) were imported and
 measured. None was a drop-in; the full results are in
-[`docs/npc-model-sourcing.md`](../npc-model-sourcing.md).
+[`docs/assets/npc-model-sourcing.md`](../assets/npc-model-sourcing.md).
 
 ## Decision
 
@@ -86,4 +86,4 @@ measured. None was a drop-in; the full results are in
 - MakeHuman is now an active local development pipeline (body retarget plus project-owned
   head/hands), while the procedural library remains the shipped path and CI fallback.
 - Blender MCP is optional interactive tooling only; it is not a build, CI, test or runtime
-  dependency. Setup is in [`docs/asset-tooling.md#optional-interactive-blender-mcp`](../asset-tooling.md#optional-interactive-blender-mcp).
+  dependency. Setup is in [`docs/assets/asset-tooling.md#optional-interactive-blender-mcp`](../assets/asset-tooling.md#optional-interactive-blender-mcp).

docs/adr/0017-quaternius-npc-base.md

@@ -16,7 +16,7 @@ The CC0 **Quaternius Ultimate Modular Characters** pack already provides a
 coherent low-poly human silhouette, 21 modular outfits and 24 animation clips.
 The researched GLBs (`Casual`, `Casual_Hoodie`, `Worker`) measure 4 meshes, 62
 joints and 24 clips each. The owner directed that Quaternius become the NPC
-base. The full evaluation is in [`docs/npc-model-sourcing.md`](../npc-model-sourcing.md).
+base. The full evaluation is in [`docs/assets/npc-model-sourcing.md`](../assets/npc-model-sourcing.md).
 
 ## Decision
 

docs/adr/0018-quaternius-stage2-runtime-rig.md

@@ -10,7 +10,7 @@ ADR-0017 made Quaternius the NPC base and staged the migration: Stage 1 bakes a
 proportion-warped mesh onto the existing 13-core contract; Stage 2 adopts the
 62-joint rig at runtime only if the warp fails.
 
-The Stage 1 proportion warp was executed and measured (`docs/npc-model-sourcing.md`
+The Stage 1 proportion warp was executed and measured (`docs/assets/npc-model-sourcing.md`
 §Rest-pose and proportion-warp measurement). The arm-down rest pose bakes cleanly, but the
 warp to the contract proportions does not: