Worldgen Phase 0: add terrain modifier tests

[MichaelFisher1997]

Parent: #646

Phase

Phase 0: Baseline and safety

Goal

Add tests for terrain modifier semantics before wiring modifiers into the live terrain pipeline.

Tasks

• Define expected behavior for height_amplitude.
• Define expected behavior for smoothing.
• Define expected behavior for clamp_to_sea_level.
• Define expected behavior for height_offset.
• Include edge cases near sea level and high elevation.

Acceptance Criteria

• Tests document modifier semantics clearly.
• Tests are deterministic and do not require chunk rendering.
• Existing tests still pass with nix develop --command zig build test.

Parallelization

Can be done in parallel with Phase 0 baseline/reporting work.

[github-actions]

Based on codebase analysis, here's my assessment:

Classification: Feature Request (Test Addition)

Finding: The issue contains empty placeholder bullets ("Define expected behavior for .") making it unclear which specific modifiers need tests. However, the TerrainModifier struct in modules/world-worldgen/src/biome_registry.zig:56-65 has exactly 4 fields that would be the likely targets:

1. height_amplitude (f32) - hill/mountain amplitude multiplier
2. smoothing (f32) - terrain flattening factor (0=none, 1=fully flat)
3. clamp_to_sea_level (bool) - forces terrain to sea level
4. height_offset (f32) - additive height adjustment

Current State: No dedicated TerrainModifier tests exist. Tests in biome_registry_tests.zig only indirectly verify modifier values via biome comparisons.

Suggestion: The empty bullets likely represent these 4 fields. Consider:

• Adding terrain_modifier_tests.zig following the existing pattern in biome_registry_tests.zig
• Testing edge cases: sea level (y=62), high elevation boundaries, amplitude=0, smoothing=1, combined modifiers

Question for author: The bullet points appear empty. Can you confirm the 4 fields above are the intended test targets, or are there additional/different modifiers to test?

opencode session  |  github run

[github-actions]

This is a Phase 3 validation task. Based on codebase analysis:

Current Integration:

• region.zig:50-75 - Role-based height/vegetation multipliers
• region.zig:78-111 - Hard feature gates (allowLake, allowHeightDrama, etc.)
• height_sampler.zig:249 - Uses controls.height_mult for terrain modulation
• overworld_generator.zig:974 - Stores region_role in classification cache

Key Design Observation:

Biome selection (biome_source.selectBiome at terrain_shape_generator.zig:265) uses pure climate/structural params and does not receive RegionControls. Region controls affect terrain characteristics (height, vegetation) and feature gates, but biome ID selection operates on a separate layer and cannot conflict with region controls by design.

This should be verified explicitly. If the intent was to have region role bias biome selection, that mechanism is currently missing.

Suggested Verification:

1. Run -Dstartup-diagnostic-seconds=5 for chunk/LOD counts and region role sampling
2. Sample transit vs destination regions for height and vegetation density statistics
3. Confirm mountain destinations have drama_mask > 0 applied at height_sampler.zig:261
4. Boundary regions should have height_mult=1.0 per region.zig:59

Gap: No baseline/report coverage mechanism exists for region-role effects. Adding this would require extending WorldClassMap or adding telemetry to track per-role terrain statistics.