test(M00): add checkpoint perceptual comparison

Work-Package: M00-QAR-VISUAL-DIFF-001
Agent: sindo-main-codex
Tests: synthetic visual diff, M00 dry-run, coordination and documentation gates
Fidelity: automates paired metrics; approved 3.3.5a captures and human approval remain required

coordination/claims/M00-QAR-VISUAL-DIFF-001.md

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+# M00-QAR-VISUAL-DIFF-001 — Renderer checkpoint visual diff
+
+<!-- OPENWC_CLAIM:M00-QAR-VISUAL-DIFF-001:sindo-main-codex:2026-07-13 -->
+
+## Ownership
+
+- Target: M00
+- Program: QAR
+- Owner/Agent ID: sindo-main-codex
+- Branch: `work/sindo-main-codex/m00-checkpoint-diff`
+- Lease expires UTC: 2026-07-13
+- Integrator: milestone integrator
+
+## Outcome
+
+Provide a deterministic, headless perceptual comparison for paired renderer checkpoint PNGs, with machine-readable results and synthetic regression coverage.
+
+## Non-goals
+
+- Supplying or committing original-client screenshots.
+- Claiming visual parity or replacing human approval.
+- Image registration, camera correction, or temporal animation matching.
+
+## Paths
+
+- Exclusive: `src/tools/compare_render_checkpoints.gd`
+- Shared/hotspots: `src/tools/render_baseline_manifest.json`, `tools/run_render_baseline.ps1`, `docs/RENDER_BASELINE.md`, `docs/modules/world-renderer.md`, `RENDER.md`
+- Generated/ignored: comparison reports, diff PNGs, original-client captures
+
+## Contracts and data
+
+- Public API/events: headless CLI arguments and JSON report documented in renderer baseline docs
+- Schema/format version: comparison report schema 1; baseline manifest schema remains 1
+- Migration/compatibility: additive manifest budget fields
+- Consumers: developers and CI
+
+## Dependencies
+
+- Requires: Godot `Image` API and M00 checkpoint naming contract
+- Blocks: automated portion of original-client paired comparison
+- External state: approved build 12340 screenshots remain unavailable
+
+## Verification
+
+- Commands: headless self-test; manifest verification; documentation and coordination gates
+- Fixtures: tool-generated identical and changed 2x2 PNGs
+- Fidelity evidence: algorithm is deterministic; real-client evidence remains a manual/external input
+- Performance budget: offline checkpoint operation, linear in pixel count
+
+## Documentation deliverables
+
+- Inline public API docs: CLI usage in script header
+- Module specification: update renderer verification and source map
+- Data-flow diagram: update baseline documentation
+- Sequence/state/dependency diagrams: not stateful or asynchronous; not applicable
+- Source map/status updates: renderer module and RENDER notes
+
+## Simplicity and naming
+
+- Important names introduced: `mean_perceptual_error`, `changed_pixel_ratio`
+- Simplest considered solution: direct Godot Image iteration with no dependency
+- Rejected complexity/abstractions: external image library, SSIM window pipeline, image registration
+- Unavoidable complexity and justification: sRGB linearization avoids comparing encoded channel values directly
+- Measured optimization evidence: not required for offline seven-checkpoint comparison
+
+## Status
+
+- State: ready
+- Done: comparator, runner integration, manifest tolerance contract, synthetic regression, module/baseline documentation, repository gates
+- Next: integrator review and paired run when approved build 12340 screenshots become available
+- Blocked by: real paired screenshots only for final human fidelity approval
+
+## Handoff
+
+- Commit: branch HEAD
+- Results: synthetic identical image passed, changed image failed; full M00 dry-run and repository gates passed
+- Remaining risks: thresholds need calibration against approved paired captures; animation phase and camera alignment still require human review
+- Documentation updated: `docs/RENDER_BASELINE.md`, `docs/modules/world-renderer.md`, `RENDER.md`