[Bug] SDK primitive materials (box/plane) don't match Unity rendering (mobile)

## **⚠️ Problem:**
Content team reports SDK primitives (planes, cubes) created via code render with different color/shading in Godot (mobile) vs Unity Explorer. Undermines cross-client visual parity for creators shipping scenes to both platforms; no player-facing metric number available yet.

## **🏁 Scope:**
Reproduce with a minimal scene (default-material box/plane authored via `MeshRenderer`/`Material` SDK components, no GLB), compare Godot vs Unity render, root-cause and fix the divergence. Bounded to code-authored primitive shapes — not GLB/imported-model materials.

## **📝 Deliverables:**
- [ ] Minimal repro scene (code-created box + plane, default/simple material)
- [ ] Side-by-side screenshot comparison Godot vs Unity documented on the issue
- [ ] Root cause identified (color space, lighting response, PBR defaults, etc.)
- [ ] Fix shipped closing the visual gap
- [ ] Regression check against other primitive-material reports (#2719, #2730) to confirm shared or separate root cause

## **🔗 References:**
- [decentraland/godot-explorer#2719](https://github.com/decentraland/godot-explorer/issues/2719) — confirmed root-caused UV tiling/offset V-direction bug on primitive materials; candidate shared cause, unconfirmed
- [decentraland/godot-explorer#2730](https://github.com/decentraland/godot-explorer/issues/2730) — same class of report (plane shader/color diff vs Unity), single-scene repro
- [decentraland/godot-explorer#1807](https://github.com/decentraland/godot-explorer/issues/1807) — older, broader color/backface parity report; flagged by requester as stale/unrelated, not treated as duplicate here but same symptom family

## Notes
No screenshot or scene reference yet from content team — requested, will attach when available.

**Requested by Manuel Maceira via Slack**