perf(web): share card texture via shader UV transform

Move the card sprite repeat/offset out of per-card texture clones and
into a uniform injected into the material shader. Cards now share the
deck sheet (one GPU upload), the shader, and the geometry, with only
per-card material uniforms differing, so navigating a deck no longer
re-uploads the sheet on every step.

docs/decisions.md

@@ -299,4 +299,28 @@ flips.
 
 **Alternatives considered:** Reporting only a hit and silently dropping
 misses. Rejected — a command that needs to react to a wrong tap has no way to
-do so. World-space trigger points. Rejected — they break when the part moves.
+do so. World-space trigger points. Rejected — they break when the part moves.
+
+## D21 — Card sprite UVs live in the material shader, not the texture
+
+**Decision:** A card's sprite cell is selected by a repeat/offset injected into
+the material's shader (`cardMaterial.ts` extends `MeshStandardMaterial` via
+`onBeforeCompile`) rather than by cloning the texture and setting its
+`repeat`/`offset`.
+
+**Context:** Cards in a deck share one sprite sheet (drei caches the texture by
+URL), but each card samples a different cell. The previous approach cloned the
+texture per card to set its UVs; each clone gets its own WebGL texture binding,
+so navigating a deck re-uploaded the whole sheet on every step. Moving the
+transform into a per-material uniform lets cards share the texture (one GPU
+upload), the shader (identical injected source → one program), and the geometry,
+with only the material uniforms differing.
+
+We inject our own uniform rather than setting `texture.repeat`/`offset` because
+three r185 derives map UVs from a `mapTransform` matrix refreshed from
+`map.matrix` every frame, which would overwrite a per-material transform set on
+the shared texture.
+
+**Alternatives considered:** Cloning the texture per card (previous approach).
+Rejected — re-uploads the sheet per card. A module-level cache of per-card
+clones. Rejected — still one upload per unique card instead of one per sheet.