Worldgen: biome terrain modifiers and multi-parameter biome selection

[MichaelFisher1997]

Goal

Make terrain shape respond to biome metadata and move biome selection from heat/humidity-only Voronoi toward a richer multi-parameter climate model.

Why

The worldgen pipeline already has useful biome metadata and Minecraft/Minetest-inspired climate inputs, but several fields are not yet used by terrain generation. This work should make biomes more structurally distinct, improve long-distance biome distribution, and keep LOD/worldgen behavior deterministic.

Scope

• Apply BiomeDefinition.terrain modifiers to terrain shaping.
• Extend biome selection beyond heat/humidity into continentalness, erosion/ruggedness, peaks/valleys, and elevation.
• Preserve existing ocean, coast, river, transition, and LOD behavior unless intentionally retuned.
• Add baseline reports/tests before tuning.

Phase 0: Baseline And Safety

These can run in parallel.

• Worldgen Phase 0: add biome and height distribution baselines #647
• Worldgen Phase 0: add biome selector regression tests #648
• Worldgen Phase 0: add terrain modifier tests #649

Phase 1: Terrain Modifier Integration

The integration issue is the serial dependency. The tuning issues can run in parallel after it lands.

• Worldgen Phase 1: apply BiomeDefinition TerrainModifier during height shaping #650
• Worldgen Phase 1: tune wetland biome terrain modifiers #651
• Worldgen Phase 1: tune mountain biome terrain modifiers #652
• Worldgen Phase 1: tune dry biome terrain modifiers #653

Phase 2: Multi-Parameter Biome Selection

The selector data-model issue is the serial dependency. Migration, priority rules, and LOD consistency can run in parallel after the shape is in place.

• Worldgen Phase 2: introduce multi-parameter biome points #654
• Worldgen Phase 2: migrate existing biome points to multi-parameter scoring #655
• Worldgen Phase 2: preserve ocean, coast, and river priority rules #656
• Worldgen Phase 2: keep LOD biome selection consistent with LOD0 #657

Phase 3: Compatibility And Tuning

These can run mostly in parallel after Phase 2 starts producing stable outputs.

• Worldgen Phase 3: retune biome adjacency and transition bands #658
• Worldgen Phase 3: validate region role interaction with biome selection #659
• Worldgen Phase 3: add visual and debug snapshots for worldgen tuning #660

Phase 4: Polish

These happen after core implementation stabilizes. Documentation can start once the implementation shape is mostly final.

• Worldgen Phase 4: tune default seed experience #661
• Worldgen Phase 4: document biome selection and terrain modifier pipeline #662

Acceptance Criteria

• Seeded worldgen remains deterministic.
• Existing unit tests pass with nix develop --command zig build test.
• New tests cover modifier application and selector behavior.
• LOD biome/height output remains consistent with LOD0 enough to avoid obvious seams.
• The new pipeline is documented enough for future biome additions.