feat(report): add text extraction health check

[MMoney1988]

Summary

• Adds a warning-only Text Extraction Health check to the Markdown quality report.
• The check measures extracted-text availability in normalized parser-output blocks.
• Regenerates existing MDPI example reports and updates README/test coverage.

Why

• PQR already checks required fields, provenance, bounding boxes, layout/noise, and local text usefulness.
• It did not yet summarize whether the normalized output contains enough extracted text overall for human review.
• This improves parser-output review without claiming OCR support or OCR accuracy.

What

• Adds Text Extraction Health after Text Usefulness.
• Reports text-bearing block counts, non-empty text blocks, empty text blocks, total text characters, image blocks, and empty-text ratio.
• Keeps the check warning-only: no new hard-failure path and no CLI exit-code change for warnings.
• Adds conservative report interpretation and README wording that distinguish text availability from text correctness.
• Adds tests for PASS/WARN paths, warning-only behavior, CLI exit code, and conservative interpretation.

Tests

• .venv/bin/python -m pytest -p no:cacheprovider tests -> 62 passed
• .venv/bin/python -m ruff check src tests -> passed
• git diff --check -> passed
• .venv/bin/python -m pdf_quality_report.report --input examples/mdpi_pdf_elements/page_006/normalized_blocks.json --output /tmp/page_006_report.md -> passed
• .venv/bin/python -m pdf_quality_report.report --input examples/mdpi_pdf_elements/page_012/normalized_blocks.json --output /tmp/page_012_report.md -> passed
• Claim scan for OCR/support/readiness terms found only the existing negative README boundary: does not claim OCR quality.

Risks

• Thresholds are conservative heuristics for review signals, not correctness guarantees.
• The check can move an otherwise GO report to REVIEW, but it cannot produce BLOCK.
• This PR does not add OCR fixtures, OCR benchmarking, invoice support, RAG, parser adapters, or product-readiness claims.

Rollback

• Revert commit a085676 to remove the check, README/test updates, and regenerated example reports.