feat: add column span LOD mesh builder

[MichaelFisher1997]

Summary

• Add a public column/span LOD mesh builder that consumes vertical span data and falls back to the stable heightfield path when spans are unavailable.
• Emit top faces, exposed vertical side faces, and water spans while using adjacent column coverage to avoid unnecessary internal edge faces.
• Add tests for steep terrain, water spans, empty columns, span ordering, and fallback behavior.

Verification

• nix develop --command zig fmt modules/world-lod/src/lod_mesh.zig
• nix develop --command zig build test -- --test-filter "buildFromColumnSpans"
• nix develop --command zig build test

Fixes #689

[github-actions]

📋 Summary

This PR implements Phase 3 of issue #689: a new column/span LOD mesh builder that consumes rich vertical span data and falls back to the existing heightfield path when spans are unavailable.

Linked Issue: #689 — "Phase 3: add a column/span LOD mesh builder"

Verification of Issue Requirements:

• ✅ New mesh builder implemented alongside existing heightfield builder
• ✅ Generates top faces, exposed side faces, and water spans
• ✅ Uses adjacent column coverage to avoid unnecessary internal edge faces
• ✅ Heightfield fallback preserved when spans are absent
• ✅ Tests added for steep terrain, water, empty columns, span ordering, and fallback
• ⚠️ Greedy/local quad merging not implemented (listed as optional in issue)

All acceptance criteria from #689 are satisfied. The implementation is clean, well-tested, and correctly integrated with existing LOD mesh infrastructure.

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📌 Review Metadata

• Reviewed Commit SHA: 49821e65cae38f3a73d95be5ddbb5833b44c5972
• Reviewed PR: feat: add column span LOD mesh builder #695
• Linked Issues: Phase 3: add a column/span LOD mesh builder #689

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🔴 Critical Issues (Must Fix - Blocks Merge)

None identified.

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⚠️ High Priority Issues (Should Fix)

None identified.

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💡 Medium Priority Issues (Nice to Fix)

None identified.

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ℹ️ Low Priority Suggestions (Optional)

None identified.

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📊 SOLID Principles Score

Principle	Score	Notes
Single Responsibility	8	buildFromColumnSpans delegates to focused helpers (collectColumnSpans, addExposedSpanFaces, subtractCoveredInterval)
Open/Closed	8	New path added without modifying existing heightfield/QEM builders
Liskov Substitution	9	Same interface contract as buildFromSimplifiedData; callers can swap paths
Interface Segregation	9	Public API is a single method; internal helpers are private
Dependency Inversion	8	Depends on existing LODSimplifiedData and TextureAtlas abstractions
Average	8.4

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🎯 Final Assessment

Overall Confidence Score: 92%

Confidence Breakdown:

• Code Quality: 92% (Clean, idiomatic Zig with proper allocator patterns and mutex usage)
• Completeness: 95% (Fully satisfies issue Phase 3: add a column/span LOD mesh builder #689 requirements; tests cover all stated scenarios)
• Risk Level: 85% (New code path is opt-in/test-only; no existing behavior modified)
• Test Coverage: 95% (5 targeted tests + full suite passing)

Merge Readiness:

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

Verdict:

MERGE

Clean, well-tested implementation that fully addresses #689 with no blocking issues.

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  "reviewed_sha": "49821e65cae38f3a73d95be5ddbb5833b44c5972",
  "critical_issues": 0,
  "high_priority_issues": 0,
  "medium_priority_issues": 0,
  "overall_confidence_score": 92,
  "recommendation": "MERGE"
}

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