feat: apply biome terrain modifiers

[MichaelFisher1997]

Summary

• Select a stable preliminary biome from unmodified terrain before applying biome terrain modifiers.
• Apply biome height_amplitude, smoothing, clamp_to_sea_level, and height_offset during height shaping while keeping ocean decisions hard and peak compression active.
• Add height sampler coverage for flattening/clamping, amplification, and peak compression, and update deterministic worldgen fingerprints.

Verification

• nix develop --command zig fmt modules/world-worldgen/src/height_sampler.zig modules/world-worldgen/src/terrain_shape_generator.zig src/worldgen_tests.zig
• nix develop --command zig build test

Fixes #650

[github-actions]

📋 Summary

This PR implements Issue #650: applying BiomeDefinition.terrain modifiers during height generation. The implementation uses a stable two-pass approach:

1. Select a preliminary biome from unmodified terrain
2. Re-sample with the biome's TerrainModifier (height_amplitude, smoothing, clamp_to_sea_level, height_offset)
3. Apply peak compression after modifiers

The PR correctly avoids circular instability by anchoring biome selection to unmodified terrain. Ocean decisions remain hard (continentalness-based), and peak compression is preserved post-modifier. Tests cover flattening, amplification, clamping, and peak compression interaction. Deterministic worldgen fingerprints are updated to reflect the new generation pipeline.

📌 Review Metadata

• Reviewed Commit SHA: 8fb6a88af0276152e43e05871081b1e64e19752a
• Reviewed PR: feat: apply biome terrain modifiers #669
• Linked Issue: Worldgen Phase 1: apply BiomeDefinition TerrainModifier during height shaping #650 - fully addressed. All acceptance criteria are met:
  • Swamp-like modifiers can flatten/clamp terrain near sea level
  • Mountain-like modifiers can amplify terrain without breaking peak compression
  • Generation remains deterministic for fixed seeds (fingerprints updated)
  • Tests pass

🔴 Critical Issues (Must Fix - Blocks Merge)

✅ All previously reported critical issues have been resolved.

None identified.

⚠️ High Priority Issues (Should Fix)

✅ All previously reported high priority issues have been resolved.

None identified.

💡 Medium Priority Issues (Nice to Fix)

None identified.

ℹ️ Low Priority Suggestions (Optional)

[LOW] modules/world-worldgen/src/terrain_shape_generator.zig:281-297 - Minor code duplication in biome selection
Confidence: High
Description: prepareChunkPhaseData manually constructs ClimateParams and StructuralParams to call selectBiome, duplicating logic already encapsulated in selectBiomeForColumn (line 199).
Impact: Maintenance overhead if biome selection logic changes in the future.
Suggested Fix: Construct a ColumnData from phase_data fields and call self.selectBiomeForColumn(column, phase_data.slopes[idx]) instead.

📊 SOLID Principles Score

Principle	Score	Notes
Single Responsibility	8	HeightSampler handles height computation; TerrainShapeGenerator orchestrates the two-pass biome/modifier flow cleanly
Open/Closed	7	Adding new modifier types requires extending TerrainModifier and applyHeight, but the existing pipeline doesn't need changes
Liskov Substitution	9	computeHeight delegates to computeHeightWithTerrainModifier with null; backward compatibility preserved
Interface Segregation	9	New computeHeightWithTerrainModifier adds optional parameter without breaking existing callers
Dependency Inversion	8	TerrainModifier is a simple data struct passed into height computation; no tight coupling
Average	8.2

🎯 Final Assessment

Overall Confidence Score: 92%

Confidence Breakdown:

• Code Quality: 90% (clean implementation, minor duplication in pass 3)
• Completeness: 95% (fully addresses issue Worldgen Phase 1: apply BiomeDefinition TerrainModifier during height shaping #650 requirements, good test coverage)
• Risk Level: 85% (two-pass sampling doubles noise cost per column; acceptable for correctness)
• Test Coverage: 95% (tests for flattening, amplification, clamping, peak compression; fingerprints updated)

Merge Readiness:

• All critical issues resolved
• SOLID average score >= 6.0
• Overall confidence >= 60%
• No security concerns
• Tests present and passing

Verdict:

MERGE

Clean implementation of terrain modifier integration with stable two-pass biome selection, comprehensive tests, and passing build.

{
  "reviewed_sha": "8fb6a88af0276152e43e05871081b1e64e19752a",
  "critical_issues": 0,
  "high_priority_issues": 0,
  "medium_priority_issues": 0,
  "overall_confidence_score": 92,
  "recommendation": "MERGE"
}

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