docs: update mesh, viewers, and selection decisions

Document the separated front/back/walls extrusion, material-level back
flips, CardID sprite selection from the parent deck, index-path tree
selection, and the Bounds-inside-Suspense camera fit.

docs/implementation-plan.md

@@ -205,16 +205,20 @@ Isomorphic analysis of a parsed `TTSMod`. No Node-specific APIs.
     `scaleShape` and `signedArea`.
 - `tessellate.ts`
   - `triangulate(shape)` — earcut triangulation (same as three.js).
-  - `capFaces(shape, height, uvScale, uvBounds)` — top/bottom faces. UVs map
-    the shape's bounding box (or `uvBounds` framing) to the unit square; the
-    bottom face uses the same planar xy mapping as the top (no mirror).
+  - `frontFaces(shape, height, uvScale, uvBounds)` — top face, normal +Z.
+  - `backFaces(shape, height, uvScale, uvBounds)` — bottom face, normal -Z.
+    UVs map the shape's bounding box (or `uvBounds` framing) to the unit
+    square; the back uses the same planar xy mapping as the front (no mirror).
+  - `capFaces(...)` — front + back merged into one geometry (front vertices
+    first, then back).
 - `walls.ts`
   - `wallFaces(shape, height, uvScale, uvBounds)` — side walls with outward
     normals and planar xy UVs (z-independent).
 - `extrude.ts`
-  - `extrudeShape(shape, options)` — merged caps + walls as one geometry.
-  - `extrudeShapeParts(shape, options)` — caps and walls as separate
-    geometries (for distinct materials).
+  - `extrudeShape(shape, options)` — merged front + back + walls as one
+    geometry.
+  - `extrudeShapeParts(shape, options)` — `{ front, back, walls }` as separate
+    geometries, so each face can carry its own material.
   - `ExtrudeOptions` — `height`, `capUvScale`, `wallUvScale`, `uvBounds`.
 - `types.ts`
   - `FaceGeometry`, `ExtrudedGeometry`, `UVBounds`.
@@ -278,7 +282,8 @@ proxy API and `packages/extract` directly for analysis.
 - `components/SearchResults.tsx` — result grid + pagination.
 - `components/ObjectTree.tsx` — recursive tree sidebar; each entry shows a
   class icon (hover for the class name) + display label, indented by depth.
-  Clicking selects an object.
+  Clicking selects an object by its unique index path (not GUID — cards in a
+  deck share the deck's GUID).
 - `components/viewers.tsx` — viewer registry (`registerViewer` /
   `resolveViewer`) plus a `DefaultViewer` that renders an object's fields;
   custom per-class viewers can be registered later.
@@ -286,13 +291,19 @@ proxy API and `packages/extract` directly for analysis.
   `@react-three/drei`, and `@react-three/postprocessing`. `register.ts`
   registers lazy-loaded viewers for `Tile`/`Custom_Tile` (flat box),
   `Custom_Token` (shape traced from the image's alpha channel via `/trace`,
-  extruded with `@tts/mesh`), `Card`/`Deck`/`Custom_Deck` (thin box with
-  face/back textures), and `Custom_Model`/`Custom_Model_Bag`/
-  `Custom_Model_Infinite_Bag` (GLTF/OBJ/FBX from `CustomMesh.MeshURL`).
-  `Scene.tsx` is a shared canvas with lighting, orbit controls, contact
-  shadows, and subtle bloom/vignette. `assetUrl.ts` routes asset URLs through
-  the proxy for CORS-safe loading. The viewers are lazy-loaded so the three.js
-  stack is code-split out of the main bundle.
+  extruded with `@tts/mesh`), `Card`/`CardCustom`/`Deck`/`DeckCustom`/
+  `Custom_Deck` (thin rounded rect with face/back textures), and
+  `Custom_Model`/`Custom_Model_Bag`/`Custom_Model_Infinite_Bag` (GLTF/OBJ/FBX
+  from `CustomMesh.MeshURL`). Viewers build `{ front, back, walls }` geometry
+  via `extrudeShapeParts`; back faces are flipped left/right on the material
+  (`flipTexture.ts`) so they aren't mirrored, and card faces slice the deck
+  sprite sheet via `CardID` (`cardResolution.ts`). `Scene.tsx` is a shared
+  canvas with lighting, orbit controls, contact shadows, and subtle
+  bloom/vignette; it fits the camera to the object's bounds via drei's
+  `Bounds` inside the Suspense boundary, so it frames the loaded content.
+  `assetUrl.ts` routes asset URLs through the proxy for CORS-safe loading.
+  The viewers are lazy-loaded so the three.js stack is code-split out of the
+  main bundle.
 - `components/objectIcons.tsx` — maps TTS object classes to one or more
   Iconify icons (`iconsForObject`); unknown classes fall back to a help icon.
   Icons may come from multiple sets (mdi, material-symbols, file-icons, ...);