feat(agent): add result-verifier for blind visual comparison

Root cause: test-runner was giving overly optimistic results due to:
1. Context bias - knew the implementation, tended to defend it
2. No actual visual comparison - just wrote 'ACCEPTABLE' without looking
3. No structural validation - accepted 35x scale differences as 'acceptable'

Solution:
- New result-verifier agent that performs blind visual comparison
- Strict pass/fail criteria for structural validation
- Updated test-runner to use result-verifier for each figure
- Clear guidelines: structural mismatches = FAIL, not ACCEPTABLE

Test result: verifier correctly identified Fig3 as FAIL with 7 specific issues:
- Wrong X-axis variable (channels vs power)
- Wrong Y-axis scale (5x difference)
- Wrong curve count (5 vs 4)
- etc.

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