[SOLID] [M2] Extract LOD-coloring and tree-hints from overworld_generator.zig

[MichaelFisher1997]

Part of #776 — SOLID architecture debt umbrella.
Phase: P1 · Finding: M2

Problem

modules/worldgen-overworld/src/overworld_generator.zig (1,107 LOC) delegates terrain/biome to collaborators but inlines LOD color computation (biomeGrassTint/biomeFoliageTint/biomeWaterTint, 662-725), tree-hint computation (computeChunkTreeHints/choosePlacedTree/placeTreeOccupancy, 492-621), and material layering (makeMaterialLayers/surfaceTypeForBlock, 884-931).

Evidence

(Full evidence and file:line citations are in the umbrella #776.)

Fix

Extract two collaborators:

• lod_coloring.zig (biomeGrassTint, biomeFoliageTint, biomeWaterTint)
• tree_hints.zig (computeChunkTreeHints, choosePlacedTree, placeTreeOccupancy)

Material layering can stay or move to surface_builder.zig.

Verification

• nix develop --command zig build test (includes shader validation)
• If rendering-touching: capture headless screenshot baseline before/after (-Dskip-present)
• If behavior-preserving refactor: golden-output test must stay green

Constraints

• Small reviewable PR targeting dev; conventional commit (refactor: / feat:)
• Preserve current behavior unless this issue explicitly changes it
• Keep worker-thread RHI isolation intact (no RHI calls off main thread)

Notes

v1 is otherwise well-decomposed (P6 strength) — this completes that decomposition.

Tracking: #776