[Phase 1A] Rendering: Add flat quad and tall cross shapes

[MichaelFisher1997]

Part of #616.

Scope

Add the simple render shapes needed by plant and layer-style blocks without taking on face-attached blocks or full custom geometry.

Target shapes:

• flat_quad, or equivalent, for thin horizontal quads such as floor vegetation or layer-like blocks.
• tall_cross, or equivalent, for two-block-high vegetation.

Acceptance Criteria

• RenderShape supports the new simple shapes, or the codebase has an equivalent data-driven representation.
• Meshing/rendering handles the shapes with existing atlas/material behavior.
• The implementation keeps worker-thread meshing free of RHI/window calls.
• nix develop --command zig build test passes.

Parallelization

Can run in parallel with #621, #622, and Phase 1B.

[github-actions]

Analysis Comment:

This is a Feature Request to extend the block shape system.

Current State:

• RenderShape enum in modules/world-core/src/block_registry.zig:31-36 has only cube and cross
• cross_mesher.zig already handles the existing X-shaped cross for single-block plants (flowers, saplings)
• tall_grass currently uses .cross but its 2-block height is handled by emitting at consecutive Y levels

Proposed Implementation:

1. Add new variants to RenderShape enum (block_registry.zig:31-36):

   • flat_quad — single horizontal quad (for carpet, snow layers, floor vegetation)
   • tall_cross — 2-block-high cross (for tall plants that need full X at bottom AND top)

2. Add meshers in modules/world-meshing/src/meshing/:

   • flat_quad_mesher.zig — similar to cross_mesher.zig but emits a single horizontal quad instead of 2 diagonals
   • tall_cross_mesher.zig — emits an X-shaped billboard spanning 2 blocks vertically (bottom quad at y, top quad at y+1)

3. Wire into chunk_mesh.zig (buildSubchunk() around line 153) alongside existing cross mesher

Note: This is scoped appropriately — it's a single subsystem (world-meshing) with clear deliverables. Can proceed in parallel with #621, #622, and Phase 1B as noted.

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