refactor: improve verification workflow with visual comparison

Major changes:
- paper-image-extractor: Generate reference_plots.py for visual verification
- paper-director: Add image understanding checkpoint with side-by-side comparison
- paper-analyzer: Add data source labeling with reliability levels
- code-writer: Change from TDD to VDD (Verification-Driven Development)
- test-runner: Generate comparison reports with images and explanations
- verification skill: Add difference classification system
- code-generation skill: Emphasize result independence

Key principles:
- Code results are authoritative, paper values are references
- Differences are expected and documented, not bugs to fix
- Visual comparison prioritized over exact numerical match
- Tests verify sanity (shape, gradient, range), not exact values

.opencode/skills/verification/SKILL.md

@@ -7,10 +7,27 @@ description: Use when verifying replication results against paper's reported val
 
 ## Overview
 
-Systematic approach to verifying that replicated code produces results matching the original paper.
+Systematic approach to verifying that replicated code produces results comparable to the original paper. **Note**: Exact matches are rare; the goal is verifiable, explainable results.
 
 **Announce at start:** "I'm using the verification skill to validate replication accuracy."
 
+## Core Philosophy
+
+1. **Code results are authoritative** - Our implementation's output is ground truth
+2. **Paper values are references** - Used for comparison, not as test assertions
+3. **Differences require explanations** - Not fixes (unless clearly buggy)
+4. **Visual comparison over numerical** - Trends matter more than exact values
+
+## Difference Classification System
+
+| Status | Symbol | Criteria | Action |
+|--------|--------|----------|--------|
+| MATCH | ✅ | < 2% difference | Document, no action needed |
+| ACCEPTABLE | ⚠️ | 2-10% difference | Document with brief explanation |
+| EXPLAINABLE | 📝 | > 10%, cause identified | Document cause thoroughly |
+| INVESTIGATE | 🔍 | > 10%, cause unknown | Review implementation |
+| PAPER_ISSUE | 📄 | Our results more reasonable | Document evidence |
+
 ## Verification Levels
 
 ### Level 1: Code Correctness
@@ -176,15 +193,78 @@ def compare_with_variance(
 ```markdown
 ## Verification Result: {Metric Name}
 
-**Paper Value**: {value} ± {std}
+**Paper Value**: {value} ± {std} (Source: {figure/table/text})
 **Our Value**: {value} ± {std}
 **Difference**: {absolute} ({relative}%)
 
-**Status**: MATCH | ACCEPTABLE | INVESTIGATE | MISMATCH
+**Status**: MATCH | ACCEPTABLE | EXPLAINABLE | INVESTIGATE | PAPER_ISSUE
 
 **Analysis**:
-{explanation of difference}
+{explanation of difference - required for all non-MATCH statuses}
 
 **Confidence**: {HIGH | MEDIUM | LOW}
 {reasoning for confidence level}
 ```
+
+## Visual Comparison Guidelines
+
+### Side-by-Side Figure Comparison
+
+Always present figures in side-by-side format:
+
+```markdown
+| Paper Reference | Our Replication |
+|-----------------|-----------------|
+| ![](ref_fig.png) | ![](our_fig.png) |
+```
+
+### What to Compare
+
+1. **Trends**: Does the curve go up/down at the same places?
+2. **Shape**: Is the overall shape similar?
+3. **Key points**: Do peaks/valleys occur at similar locations?
+4. **Scale**: Are values in the same order of magnitude?
+
+### Acceptable vs Unacceptable Differences
+
+**Acceptable** (document and move on):
+- Curve shifted slightly up/down (offset)
+- Slightly faster/slower convergence
+- Small noise differences
+
+**Unacceptable** (investigate):
+- Opposite trends (going up vs down)
+- Completely different shapes
+- Order of magnitude differences
+- Missing features (e.g., expected oscillation absent)
+
+## Common Difference Sources
+
+### Expected Differences (ACCEPTABLE)
+
+| Source | Typical Impact | Mitigation |
+|--------|---------------|------------|
+| Random seed | 1-3% | Run multiple seeds, report mean±std |
+| Floating point | < 0.1% | Use float64 for verification |
+| Framework differences | 1-5% | Document framework version |
+| Hardware differences | 0.5-2% | Note in report |
+| Batch size changes | 2-10% | Adjust LR proportionally |
+
+### Concerning Differences (INVESTIGATE)
+
+| Source | Typical Impact | Action |
+|--------|---------------|--------|
+| Wrong architecture | > 10% | Review code vs paper |
+| Wrong hyperparameters | 5-20% | Verify all settings |
+| Data preprocessing | Variable | Match paper exactly |
+| Bug in implementation | Variable | Debug systematically |
+
+### Paper Issues (PAPER_ISSUE)
+
+Sometimes the paper contains errors. Signs include:
+- Results that violate mathematical constraints
+- Impossible performance claims
+- Inconsistencies between text and figures
+- Known errata
+
+Document evidence thoroughly if claiming paper issue.