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 Delete the superseded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/assets/README.md

@@ -0,0 +1,19 @@
+# Asset Documentation
+
+Asset contracts, pipelines, sourcing and reviews. These pages own the measured evidence; the
+executable gates are `tools/blender_assets.py inspect|audit|fit`, `tools/npc_proportions.py` and
+the capture scripts.
+
+- [`blender-assets.md`](blender-assets.md) — Blender library operation, the character/gear/prop
+  contract, attach points and rebuild discipline.
+- [`asset-tooling.md`](asset-tooling.md) — image/model pipelines, the optional Blender MCP setup
+  and the screenshot/vision gates.
+- [`npc-model-sourcing.md`](npc-model-sourcing.md) — character-source evaluation, licence rules,
+  the Quaternius intake and the active NPC plan.
+- [`3d-model-review.md`](3d-model-review.md) — measurements, per-asset scorecard, ranked gaps and
+  accepted limits.
+- [`makehuman-head-review.md`](makehuman-head-review.md) — the retained MakeHuman fallback head,
+  its provenance and the review verdict.
+
+Up: [`../README.md`](../README.md) · Product plan: [`../../PLAN.md`](../../PLAN.md) · Tasks:
+[`../../TODO.md`](../../TODO.md).