Add 7 adversarial PDF fixtures exercising Phase 1 error-recovery paths:
- xref_30pct_bad_offsets.pdf: 100 objects, 30 bad xref offsets
- missing_mediabox_all_pages.pdf: 10 pages, no /MediaBox at any level
- missing_endobj.pdf: object 5 missing endobj marker
- truncated_mid_stream.pdf: FlateDecode stream truncated mid-decompression
- int_overflow_bbox.pdf: /BBox value 99999999999999999 (i32 overflow)
- nested_failure.pdf: every page has at least one diagnostic
- combined_failures.pdf: combines multiple failure modes (keystone INV-8 test)
Each fixture has a sibling .expected_diagnostics.json file with threshold
counts (>= not == per EC-07/EC-09 to tolerate drift).
Integration test harness (error_recovery_integration.rs):
- assert_diagnostic_count_at_least() helper for threshold checking
- assert_no_panic() helper using std::panic::catch_unwind for INV-8
- Individual test functions for each fixture
- Cumulative test_inv_8_no_panics_across_all_fixtures()
All 8 tests pass. INV-8 verified: zero panics across all fixtures.
Closes: pdftract-4w0v4
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