perf: remove LOD CPU and submission hot spots

[MichaelFisher1997]

Summary

• retain the Phase 0 LOD profiling baseline now present on stage
• share frame-keyed LOD visibility and coverage between terrain and water, with cheap rejects before coverage scans
• default-enable pooled LOD MDI for terrain and water while preserving mixed/direct fallback paths
• move cache reads, serialization, and writes to a bounded dedicated worker with token/revision validation
• mesh outside the global manager lock and reconcile paused queued jobs safely
• add per-level visibility/rejection telemetry to benchmark artifacts

Validation

• nix develop --command zig build test
• nix develop --command zig build -Doptimize=ReleaseFast
• nix develop --command zig build -Dskip-present
• bounded ReleaseFast traversal benchmark (artifact produced; local GPU-memory SLO exceeded due environment allocation reporting)
• pre-push formatting and full test hooks

Closes #920

[github-actions]

kcov coverage

Line coverage ran for this PR and uploaded a non-blocking report artifact named kcov-report. Codecov upload is configured as non-blocking while the project captures a stable baseline.

[github-actions]

📋 Summary

This PR closes #921 (LOD GPU roadmap Phase 0: establish profiling and quality baselines). It adds opt-in thread-safe CPU profiling for LOD scheduling, generation, meshing, upload, visibility/coverage, eviction, and waitIdle; adds GPU timestamp scopes for LOD terrain and water; extends benchmark artifacts with per-frame LOD attribution, percentiles, and worst-frame analysis; and introduces deterministic benchmark scenarios (stationary, traversal, rapid-turn, teleport-eviction) with documentation.

The implementation largely satisfies the issue requirements: LOD CPU/GPU timings are reported independently, frame percentiles and worst-frame attribution are available, CPU/GPU/synchronization/memory-pressure metrics are distinguished, scenarios are bounded and documented, and instrumentation can be disabled or is negligible when off. A few requested metrics (GPU pool/direct-buffer bytes, lock wait/hold time) are explicitly deferred because the RHI/LOD pool interface does not expose them.

📌 Review Metadata

• Reviewed Commit SHA: f83e2d8f04c2882b632fbefbf386c149736091cf
• Reviewed PR: perf: remove LOD CPU and submission hot spots #924
• Linked Issue: LOD GPU roadmap Phase 0: establish profiling and quality baselines #921

🔴 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:211 - LODProfilingCollector.reset() is implemented and tested but never called from production code.
Confidence: High
Description: The collector accumulates counters for the process lifetime. For long-running non-benchmark sessions, cumulative values can grow very large and become less useful for UI display over time.
Impact: UI telemetry may eventually show unbounded cumulative values instead of per-session or per-reset deltas.
Suggested Fix: Reset the collector at a sensible boundary (e.g., world load/unload) or expose a reset control if telemetry consumers need windowed measurements.

[LOW] modules/world-lod/src/lod_stats.zig:34 - Issue #921 requested GPU pool/direct-buffer bytes and pool growth/compaction tracking, which are not implemented.
Confidence: Medium
Description: The PR explicitly documents that GPU pool allocations are unavailable through the current RHI/LOD pool interface, but the issue acceptance criteria still list them.
Impact: Phase 0 is still unblocked, but later phases will need to extend the RHI/LOD pool interface or add pool-level accounting.
Suggested Fix: Capture this as a follow-up issue with a concrete interface design (e.g., add a poolBytesUsed/poolBytesReserved callback to LODGPUBridge).

[LOW] modules/world-lod/src/lod_stats.zig:34 - Lock wait/hold time is not tracked separately from waitIdle time.
Confidence: Medium
Description: Issue #921 requested lock wait/hold time, but the current instrumentation only measures waitIdle stalls.
Impact: Main-thread contention on the LOD mutex is not visible in telemetry.
Suggested Fix: Add optional Mutex wrapper timing or instrument lock/unlock pairs around the critical sections.

📊 SOLID Principles Score

Principle	Score	Notes
Single Responsibility	8	Profiling collector, benchmark runner, RHI timing, and UI display each have clear, narrow responsibilities.
Open/Closed	8	Extends existing stats/timing/benchmark structs without changing core LOD behavior.
Liskov Substitution	8	RHI interface usage is consistent; no inheritance violations.
Interface Segregation	9	LODProfilingCollector exposes only profiling-related operations; UI boundary uses a dependency-neutral projection.
Dependency Inversion	8	Benchmark/UI depend on RHI and world-lod abstractions, not concrete implementations.
Average	8.2

🎯 Final Assessment

Overall Confidence Score: 82%

Confidence Breakdown:

• Code Quality: 85% (clean, idiomatic Zig; atomic counters; clear naming)
• Completeness: 80% (most acceptance criteria met; GPU pool/lock metrics deferred)
• Risk Level: 30% (profiling-only changes; no gameplay or rendering logic changes)
• Test Coverage: 85% (new unit tests for collector, scenarios, and benchmark summaries)

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

The PR is well-scoped, well-tested, and low-risk. It establishes the requested Phase 0 baselines with clear documentation of the remaining gaps.

Machine Readable Verdict

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  "reviewed_sha": "f83e2d8f04c2882b632fbefbf386c149736091cf",
  "critical_issues": 0,
  "high_priority_issues": 0,
  "medium_priority_issues": 0,
  "overall_confidence_score": 82,
  "recommendation": "MERGE"
}

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