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 Delete the superseded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.

CREDITS.md

@@ -16,7 +16,7 @@ been cleared for public distribution.
 ## 3D characters, gear and props
 
 The local MakeHuman head prototype uses the CC0 MakeHuman/MPFB base mesh
-(see [`docs/makehuman-head-review.md`](docs/makehuman-head-review.md)); it is a
+(see [`docs/assets/makehuman-head-review.md`](docs/assets/makehuman-head-review.md)); it is a
 development prototype, not a shipped character.
 
 The character, gear and prop library is generated by project-owned scripts
@@ -53,7 +53,7 @@ CC0 1.0). It is cut, retopologised and re-weighted by project scripts; the expor
 character GLBs remain project assets. The development source files were removed
 from `tmp/` after the 2026-11-05 pivot to Quaternius. The head is retained as a
 tested fallback, not the active NPC base. See ADR-0017 and
-`docs/npc-model-sourcing.md`.
+`docs/assets/npc-model-sourcing.md`.
 
 ### Quaternius NPC base source
 
@@ -61,7 +61,7 @@ The active NPC base migration uses **Quaternius Ultimate Modular Characters** an
 **Ultimate Modular Women** (CC0 1.0, <https://quaternius.com/>). The three
 researched GLBs are development inputs and are not shipped directly; the baked,
 retargeted character GLBs are project assets. See ADR-0017 and
-`docs/npc-model-sourcing.md` for the decision, source hashes and rollout status.
+`docs/assets/npc-model-sourcing.md` for the decision, source hashes and rollout status.
 
 The venue geometry is authored procedurally in `src/game/world_builder.gd` as a
 stylized reconstruction. The Joke Shop is a fictional stand-in for the real
@@ -118,7 +118,7 @@ reference more directly, its attribution and licence must be added here.
 
 All runtime audio is procedural (crowd bed, radio crackle and interface cues).
 There are no external audio files, so there is no separate music/SFX licence to
-clear. See `docs/accessibility.md` for caption and volume behaviour.
+clear. See `docs/design/accessibility.md` for caption and volume behaviour.
 
 ## Third-party names