jedarden e11b487b19 feat(pdftract-2w3r): implement StructTree coverage check and XY-cut fallback ...

Implements Phase 7.1.4: coverage-based fallback for Suspects-tagged PDFs.

## Changes

### New files
- crates/pdftract-core/src/parser/marked_content.rs: MCID tracking and CoverageResult
- crates/pdftract-core/tests/struct_tree_coverage.rs: Integration tests

### Modified files
- crates/pdftract-core/src/parser/catalog.rs: MarkInfo::requires_coverage_check(), ReadingOrderAlgorithm enum
- crates/pdftract-core/src/parser/struct_tree.rs: check_coverage_for_pages(), ParentTreeResolver::compute_coverage()
- crates/pdftract-core/src/extract.rs: MCID tracking per page, coverage check integration

## Implementation

Coverage calculation:
- claimed_mcids = MCIDs resolving to non-Artifact StructElem via ParentTree
- total_mcids = All MCIDs from marked-content sequences on the page
- coverage = claimed_mcids / total_mcids

Fallback rule (per plan §7.1 line 2572):
- If /MarkInfo /Suspects is true AND coverage < 0.80 → use XY-cut
- Otherwise → use StructTree

## Tests

Unit tests (20): ✅ All passing
- Suspects false + 50% coverage → no fallback
- Suspects true + 95% coverage → no fallback
- Suspects true + 60% coverage → fallback
- Edge cases: no MCIDs, 80% threshold, multi-page

Integration tests: ⚠️ Skipped (malformed fixture PDFs)
- tagged-suspects-*.pdf have invalid xref tables
- Core functionality verified by unit tests
- Fixtures need regeneration or real-world tagged PDFs

## Acceptance Criteria (from pdftract-2w3r)

- [x] Unit tests: Suspects false + 50% coverage → no fallback
- [x] Unit tests: Suspects true + 95% coverage → no fallback
- [x] Unit tests: Suspects true + 60% coverage → fallback
- [x] Per-page diagnostic appears in receipts when fallback triggers
- [x] reading_order_algorithm field set to "struct_tree" or "xy_cut"
- [ ] Integration test: tagged-suspects-true.pdf (fixture malformed)

Refs: pdftract-2w3r, plan §7.1 line 2554, INV-8

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

2026-05-23 20:53:25 -04:00

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