feat(visual): basic ground tileset

[f1cklepickle]

Why

• Add a basic varied ground visual pass so the new 32 PPU baseline is represented with authored tile variation, improving readability and visual quality without coupling to snapping workflow implementation.

Proposed Change

• Create/import a small ground tile variation set compatible with the 1x1 (32px) baseline.
• Apply tiles in MainPrototype as a basic visual pass with consistent scale/import settings.
• Keep scope to visual ground content only (no placement-system behavior changes).

Acceptance Criteria

• Ground visuals show basic tile variation at correct 32 PPU scale.
• Ground tiles render consistently with no blur/stretch from import settings.

Testing / Validation

• Manual:

  • Open MainPrototype, inspect ground variation in Scene/Game view, and verify scale/readability at default camera framing.

• Related: Establish 32×32 World Grid Baseline #128