fix(harness): detect clipped shading occlusion

[developer0hye]

Summary

• Track clip_path, fill_shade, and pop_clip operations in trace order so the layout audit can recognize text hidden by a later opaque clipped shading.
• Intersect nested rectangular clips and deliberately decline non-rectangular, rotated, non-extended, translucent, or non-covering cases rather than overclaiming occlusion.
• Add focused coverage for paint order, opacity, coverage, transformed clips,
  nested clip scope, self-intersecting paths, and multiple subpaths.

Related issue

Fixes #1450

Testing

• TDD red: python3 -m unittest scripts.tests.test_compare_layout.MatchAndDiffTest.test_same_text_hidden_by_later_clipped_shading_reports_visibility_mismatch failed before the implementation with AssertionError: 0 != 1.
• python3 -m unittest scripts.tests.test_compare_layout (35 passed)
• python3 -m unittest scripts.tests.test_check_visual_pr scripts.tests.test_probe_harness scripts.tests.test_check_layout_baselines scripts.tests.test_generate_layout_baselines (118 passed)
• python3 -m unittest discover -s scripts/tests (275 passed)
• python3 scripts/compare_layout.py --json GT.pdf OUTPUT.pdf against the PR fix(docx): honor in-cell picture row floors #1407 / issue Place your event title here.docx #1219 two-page PDFs: page-1 Sensitivity:Internal changes from the false GT hidden vs output painted mismatch to hidden on both sides; page 2 remains painted on both sides. The report diff contains no other change, and the remaining layout findings stay tracked in DOCX missing-font selection changes table wrapping and placement across hosts #1409.
• git diff --check
• Documentation freshness audit: PASS

Visual impact

• No rendered PDF change
• Rendered PDF change or visual evidence added
• Reason: this changes only trace-based audit classification; office2pdf PDF generation and pixels are unchanged.

Checklist

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