[Phase 3A] Biomes: Add cold and coastal variants

[MichaelFisher1997]

Part of #616.

Scope

Implement the cold and coastal overworld biome variants once the required ice/snow/aquatic foundations exist.

Target biomes:

• Frozen Ocean
• Cold Ocean
• Stony Shore
• Snowy Beach
• Frozen River

Dependencies

Depends on #625, #626, and #624.

Acceptance Criteria

• Biome definitions are added to the modern data-driven biome system.
• Surface/filler blocks, colors, temperature/moisture behavior, and decorations match the intended biome identity.
• Existing biome generation remains stable.
• nix develop --command zig build test passes.

[github-actions]

Based on my analysis of the codebase and dependency chain:

Classification: Feature Request (Phase 3A of a phased biome implementation effort)

Current Status: This issue has 3 open dependencies that must be completed first:

• [Phase 2A] Blocks: Add core natural block pack #625 (Phase 2A): Core natural block pack - adds foundational blocks
• [Phase 2B] Aquatic: Add underwater vegetation and coral foundation #626 (Phase 2B): Underwater vegetation/coral foundation
• [Phase 1B] Worldgen: Generalize biome decoration profiles #624 (Phase 1B): Generalized biome decoration profiles

All three dependencies are still OPEN, meaning the ice/snow/aquatic foundations don't yet exist.

Implementation Path (ready when dependencies complete):

The biome system is in modules/world-worldgen/src/:

• biome_registry.zig - Primary file: add BiomePoint entries (lines 181-219) and BiomeDefinition entries (lines 225-525)
• block.zig (world-core) - Add new BiomeId variants if needed
• biome_edge_detector.zig - Add transition rules for cold/coastal biome borders
• biome_color_provider.zig - Add colors for LOD rendering

Key Parameters for Target Biomes:

Biome	Temperature	Continentalness	Surface
Frozen Ocean	0.0-0.15	0.0-0.35	gravel/packed ice
Cold Ocean	0.15-0.30	0.0-0.35	gravel
Stony Shore	0.20-0.40	0.35-0.45	stone/gravel
Snowy Beach	0.05-0.15	0.35-0.45	snow/sand
Frozen River	0.0-0.20	0.45-0.55	gravel/ice

Recommendation: This issue can proceed once #625, #626, and #624 are completed. The scope is appropriate and well-scoped for a single PR.

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