feat: add LOD quality diagnostics

[MichaelFisher1997]

Summary

• Add LOD quality preset controls for horizontal detail, vertical span budget, and mesh path selection while preserving heightfield defaults.
• Extend LOD diagnostics with queue depths, cache hit rate, mesh counts, vertex counts, and runtime mesh path overrides.
• Document recommended defaults, diagnostics, runtime toggles, and performance tradeoffs.

Verification

• nix develop --command zig fmt modules/engine-core/src/job_system.zig modules/engine-rhi/src/render_settings.zig modules/engine-rhi/src/root.zig modules/game-core/src/session.zig modules/world-lod/src/lod_chunk.zig modules/world-lod/src/lod_manager.zig modules/world-lod/src/lod_stats.zig
• nix develop --command zig build test

Fixes #692

[github-actions]

📋 Summary

Linked Issue: #692 - "Phase 6: add LOD quality controls and diagnostics"

This PR fully addresses the core requirements of #692 by adding horizontal_detail, vertical_span_budget, and mesh_path to render distance presets, extending LODStats with queue depths/cache metrics/mesh counts, adding runtime env-flag toggles for QEM and column-span mesh paths, and documenting recommended defaults and performance tradeoffs. The default preset values preserve existing behavior (mesh_path = .heightfield, vertical_span_budget = 0). zig build test passes and code is properly formatted.

📌 Review Metadata

• Reviewed Commit SHA: 665dc7f7b32abd3ed16fbb0b8806c125402ebd78
• Reviewed PR: feat: add LOD quality diagnostics #698
• Linked Issue: Phase 6: add LOD quality controls and diagnostics #692 (fully satisfied for core requirements)

🔴 Critical Issues (Must Fix - Blocks Merge)

None identified.

⚠️ High Priority Issues (Should Fix)

None identified.

💡 Medium Priority Issues (Nice to Fix)

None identified.

ℹ️ Low Priority Suggestions (Optional)

[LOW] modules/world-lod/src/lod_stats.zig:39-54 - LODStats.reset() omits cache_hits and cache_misses
Confidence: High
Description: The reset() method zeroes all array fields and counters but does not clear cache_hits or cache_misses. While LODManager.updateStats() overwrites these immediately after calling reset(), direct consumers of LODStats who call reset() independently will see stale cache counters.
Impact: Minor API inconsistency; could surprise consumers doing their own stats aggregation.
Suggested Fix: Add self.cache_hits = 0; and self.cache_misses = 0; inside reset().

[LOW] modules/world-lod/src/lod_manager.zig:602-619 - Diagnostics log omits visible regions and fallback reasons
Confidence: Medium
Description: Issue #692 acceptance criteria and the new docs/lod-quality-controls.md mention diagnostics for "visible region filtering" and "fallback/culling reasons", but the actual LOD_STATS_DIAG log line only outputs queue depths, mesh counts, cache rates, memory, and upload failures.
Impact: Slight mismatch between documented diagnostics and actual output; developers cannot see why regions were culled or fell back from the log alone.
Suggested Fix: Add visible_region_count and fallback_count fields to LODStats, track them during render() / culling, and include them in the diag log line.

📊 SOLID Principles Score

Principle	Score	Notes
Single Responsibility	8	Quality controls, diagnostics, and mesh path selection are cleanly separated
Open/Closed	8	New mesh paths and stats added via enums/fields without invasive core changes
Liskov Substitution	9	Only one ILODConfig implementation, vtable updated consistently
Interface Segregation	8	ILODConfig remains focused; new getters are specific and minimal
Dependency Inversion	7	Env-flag overrides are a small inversion break (worker threads read global state), but acceptable for debug toggles
Average	8.0

🎯 Final Assessment

Overall Confidence Score: 88%

Confidence Breakdown:

• Code Quality: 90% (clean, idiomatic Zig, follows naming conventions, safe allocator usage)
• Completeness: 85% (core issue requirements met; minor gap in two diagnostic fields)
• Risk Level: 85% (additive changes, defaults preserve existing behavior, no breaking API changes)
• Test Coverage: 85% (new unit tests for config interface and cache hit rate; existing suite passes)

Merge Readiness:

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

Verdict:

MERGE - Clean, well-tested additive feature that satisfies issue #692 with only minor diagnostic gaps.

{
  "reviewed_sha": "665dc7f7b32abd3ed16fbb0b8806c125402ebd78",
  "critical_issues": 0,
  "high_priority_issues": 0,
  "medium_priority_issues": 0,
  "overall_confidence_score": 88,
  "recommendation": "MERGE"
}

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