Add the Quaternius face/silhouette gate sheet and record the geometry measurements

docs/screens/npc-face-gate/README.md

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+# NPC face and silhouette gate — Quaternius pilot (2026-11-05)
+
+This page is the owner decision point for
+[ADR-0017](../../adr/0017-quaternius-npc-base.md) + [ADR-0018](../../adr/0018-quaternius-stage2-runtime-rig.md)
+stage 2. It answers one question: **which head ships?**
+
+`npc_casual` is now the CC0 Quaternius `Casual_Hoodie` base baked to the project contract
+(arm-down rest, five tint slots, 62-joint rig at source proportions). The procedural library with
+the MakeHuman head is unchanged and still ships for the other eleven characters.
+
+## The sheet
+
+![face gate sheet](sheet_face_gate.jpg)
+
+Rows: the Quaternius base, the procedural library with the MakeHuman head, and a procedural crowd
+variant. Columns: front, three-quarter, profile, and the crowd-distance silhouette the player
+actually sees from 8 m.
+
+Source frames: `godot --path . --script res://src/test/capture_npc_baseline.gd -- --out=res://tmp/npc-gate`.
+
+## What the sheet shows
+
+| | Head height / body | Face read at portrait | Crowd-distance read |
+| --- | --- | --- | --- |
+| Quaternius base | 0.134 | clean low-poly brows, nose, jaw, layered hair | slim, upright, clearly a person |
+| procedural + MakeHuman head | ~0.20 | oversized head, soft nose, beard slab reads as a bar | bobblehead proportions |
+| procedural crowd variant | ~0.20 | oversized head, flat cheeks | bobblehead proportions |
+
+Measured (`python3 -m pytest tools/tests/test_npc_proportions.py -q`,
+`python3 tools/npc_proportions.py`):
+
+- `npc_casual` (Quaternius): head 13.4 % of height, arm-down rest, 6,206 tris, 12,988 glTF
+  vertices, texel density 73–74 px/m on the 256² maps.
+- The procedural library: head 16–22 % of height (the stylized band in
+  `tools/npc_proportions.GUIDE`).
+
+The Quaternius head is *smaller and more realistic* than the stylized library head, which is
+exactly the trade the owner has to accept or reject. The vision judge that reviewed the earlier
+head rework reads the Quaternius face as the cleaner silhouette of the three at both distances.
+
+## The three options
+
+1. **Full swap.** Every character rebaked onto the Quaternius base in `0.9.10`; the procedural
+   library and the MakeHuman head retire to the tested fallback. Consistent look, real anatomy,
+   no further head work.
+2. **Crowd only.** Quaternius for the anonymous crowd roles, procedural + MakeHuman for the named
+   roles the player talks to. Keeps two art styles in one frame.
+3. **Add head detail.** Keep the Quaternius body and author a higher-detail head (subdivision and
+   hair cards) inside the 24,000-triangle / 24,000-vertex budget. Most work; the current head
+   already reads well at gameplay distance.
+
+## Recommendation
+
+Option 1. The Quaternius head beats the MakeHuman head at portrait distance in this sheet, and it
+costs no further authoring; the budget headroom (6.2k of 24k triangles) stays available for the
+hands and gear that the review still flags as weak.
+
+## Recorded decision
+
+Status: **awaiting owner sign-off.** Until then `npc_casual` ships as the pilot base and the other
+eleven characters stay procedural.