fix(ci): parse scientific AE metrics

[developer0hye]

Summary

• accept ImageMagick AE metrics written in decimal or scientific notation
• preserve the existing success/error status checks and zero-versus-nonzero evidence gate
• add a focused regression for the real 6.83993e+06 (0.759992) output that blocked fix(pptx): honor custom geometry transforms #1520

Related issue

Fixes #1526. Unblocks #1520.

Testing

• red: python3 -m unittest scripts.tests.test_check_visual_pr.ReferenceExporterDifferenceTests.test_decoded_pixel_delta_accepts_scientific_notation — reproduced ImageMagick could not compare the evidence
• green: the same focused command — 1 passed
• python3 -m unittest scripts.tests.test_check_visual_pr — 78 passed
• python3 -m py_compile scripts/check_visual_pr.py scripts/tests/test_check_visual_pr.py
• working-tree validation against fix(pptx): honor custom geometry transforms #1520 native evidence — Visual pull request contract is complete. for a 6.84-million-pixel GT/native difference
• git diff --check
• mandatory pre-commit documentation freshness audit: PASS

Visual impact

• No rendered PDF change
• Rendered PDF change or visual evidence added
• Reason: This changes only CI evidence-metric parsing and its unit test; conversion and rendering code are untouched.

Checklist

• Commits include a Signed-off-by line
• PR scope contains one root cause
• Remaining converter or harness deviations each reference an open issue