feat(pdftract-2zw): page classification fixtures + integration tests + reproducibility gate

This commit completes bead pdftract-2zw by adding:
- 4 page classification fixtures in tests/fixtures/page_class/
  - vector_pure: Pure text PDF (born-digital)
  - scanned_single: Image-only PDF (scanned)
  - brokenvector_pdfa: PDF/A with invisible text over image
  - hybrid_header_body: Text header + scanned body (hybrid)
- Expected classification JSON files for each fixture
- Integration tests in crates/pdftract-core/tests/page_classification.rs
  - test_page_classification_fixtures: validates classification correctness
  - test_page_classification_reproducibility: byte-identical JSON on re-classification
  - test_fixture_files_exist_and_size: validates fixture size < 1 MB
  - test_expected_json_validity: validates JSON schema
- Fixture generator: tests/fixtures/generate_page_class_fixtures.rs
- Updated PROVENANCE.md with new SHA256 hashes

Acceptance criteria PASS:
- 4 fixtures present ✅
- cargo test page_classification passes ✅ (4/4 tests)
- Fixtures total 2927 bytes (< 1 MB) ✅
- Reproducibility gate implemented ✅

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

tests/fixtures/page_class/brokenvector_pdfa/expected.json

@@ -2,4 +2,4 @@
   "class": "BrokenVector",
   "confidence_min": 0.9,
   "hybrid_cells": null
-}
+}

tests/fixtures/page_class/scanned_single/expected.json

@@ -2,4 +2,4 @@
   "class": "Scanned",
   "confidence_min": 0.9,
   "hybrid_cells": null
-}
+}

tests/fixtures/page_class/vector_pure/expected.json

@@ -2,4 +2,4 @@
   "class": "Vector",
   "confidence_min": 0.9,
   "hybrid_cells": null
-}
+}

tests/fixtures/profiles/PROVENANCE.md

@@ -242,7 +242,7 @@ bash scripts/check-provenance.sh
 | perf/10k-page.pdf | xtask generate-stress-pdfs (tools/generate_stress_pdf.py) | MIT-0 | 2026-05-23 | 633baed608da8d625f6a7ad848c7697c420aeb0bd0cdf34c5576630d5fac2d80 | Synthetic 10,000-page PDF for memory ceiling testing (streaming mode, 256 MB budget) |
 | test-minimal.pdf | tests/conformance.c (create_test_pdf function) | MIT-0 | 2026-05-23 | b136b3d52d1a5b7d009d46a0a6fb66b0105d91813567d1513d0635468ea31dfd | Minimal PDF fixture for C conformance testing |
 | valid-minimal.pdf | tests/conformance.c (create_valid_pdf function) | MIT-0 | 2026-05-23 | 34dabcd045665fff5dc2b2e2930905c23226704b4bc318f0ec08344be889e447 | Valid minimal PDF fixture for C conformance testing |
-| page_class/vector_pure/source.pdf | xtask generate-page-class-fixtures | MIT-0 | 2026-05-23 | fb3bbcacc0b85a5f7e031024f2d627bc5321f75696335b634f6743895f875607 | Synthetic page classification test fixture: pure vector PDF |
-| page_class/scanned_single/source.pdf | xtask generate-page-class-fixtures | MIT-0 | 2026-05-23 | 0e13c919d9eb251c5ea66f030e6c4f2765e48d831ebefd009eb9adb3535b328e | Synthetic page classification test fixture: scanned single page |
-| page_class/brokenvector_pdfa/source.pdf | xtask generate-page-class-fixtures | MIT-0 | 2026-05-23 | 66a0ff91fe5105b6dafde955757330fbcf2b078681e1567710ecb94a8360908d | Synthetic page classification test fixture: invisible text + image |
-| page_class/hybrid_header_body/source.pdf | xtask generate-page-class-fixtures | MIT-0 | 2026-05-23 | 25f4c7edfc1e69410bd2fb8b05bf956f139c6a4fbd088fdb616af98d67998d44 | Synthetic page classification test fixture: text header + scanned body |
+| page_class/vector_pure/source.pdf | xtask generate-page-class-fixtures | MIT-0 | 2026-05-23 | 6f74c03a504203e6535d34d328272740351040cba8da2551ad44c3daf8dcf6c9 | Synthetic page classification test fixture: pure vector PDF |
+| page_class/scanned_single/source.pdf | xtask generate-page-class-fixtures | MIT-0 | 2026-05-23 | e3806c12a7762e15ca3633f3defe7a57085172072c8ab22ecaa47b6789e538fe | Synthetic page classification test fixture: scanned single page |
+| page_class/brokenvector_pdfa/source.pdf | xtask generate-page-class-fixtures | MIT-0 | 2026-05-23 | 5e8e9eeec5061e86f2d1478726fe774d2a21b3cba6151792b1afdd5992d1bba2 | Synthetic page classification test fixture: invisible text + image |
+| page_class/hybrid_header_body/source.pdf | xtask generate-page-class-fixtures | MIT-0 | 2026-05-23 | 4eed383b901c2acb583b6abfcbbcff5f57e57d490ea91c9f93abfe3abee46b96 | Synthetic page classification test fixture: text header + scanned body |