[Phase 3C] Biomes: Add mountain variants

[MichaelFisher1997]

Part of #616.

Scope

Implement mountain overworld variants after the required natural blocks and generalized decoration support are available.

Target biomes:

• Meadow
• Grove
• Snowy Slopes
• Jagged Peaks
• Frozen Peaks
• Stony Peaks

Dependencies

Depends on #624 and #625.

Acceptance Criteria

• Biome definitions are added to the modern data-driven biome system.
• Terrain/surface/decorations visually distinguish meadow, grove, snowy slope, and peak variants.
• Existing mountain generation remains stable.
• nix develop --command zig build test passes.

[github-actions]

Based on my exploration of the codebase, here's my analysis:

Classification

Feature Request - Adding 6 new mountain biome variants to the existing data-driven biome system.

Analysis

Infrastructure Status: The biome system is well-structured for this task:

• modules/world-worldgen/src/biome_registry.zig contains BIOME_REGISTRY with all biome definitions
• Each BiomeDefinition includes climate ranges, surface blocks, vegetation profiles, and terrain modifiers
• Existing mountain biomes (mountains = 8, snowy_mountains = 9) can serve as templates
• Surface builder and decoration registry are already biome-aware

Critical Blocker: Dependencies #624 and #625 do not exist in the codebase. No references found. This feature cannot be properly implemented without understanding what those dependencies provide (likely natural blocks and generalized decoration support mentioned in the issue).

Scope: Reasonable for a single PR - 6 biomes following established patterns.

Existing Transition Infrastructure: The system already supports edge-detection micro-biomes (foothills, marsh, etc.) via TRANSITION_RULES in biome_edge_detector.zig, which could be extended for mountain variants.

Recommendation

Before implementation:

1. Clarify what issues [Phase 1B] Worldgen: Generalize biome decoration profiles #624 and [Phase 2A] Blocks: Add core natural block pack #625 reference - they appear to be unimplemented/placeholder
2. Identify which existing biome(s) the new variants should be based on for surface/terrain patterns
3. Confirm whether these are new enum values or transition-only biomes (like foothills)

The architecture supports this - no systemic changes needed, just data entries in biome_registry.zig and potentially surface/decoration tweaks in surface_builder.zig/decoration_registry.zig.

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