refactor: make Flash PDF semantic assembly stages explicit

[myhloli]

Motivation

Document orchestration, title statistics/classification and text grouping/merging have broad responsibilities, while vector-table construction combines all rejection and materialization stages in one function.

Modification

Make document text classification and inline materialization explicit stages. Split title body statistics, prototypes and classifiers, and separate text-row grouping from block assembly. Extract typed vector-track, topology and materialization stages, preserving rejection order and diagnostic fields.

This is stage 6/6 of the Flash PDF equivalence refactor, based on codex/flash-pdf-05-inline. Review and integrate stages in order.

Validation

Final validation: 1,269 tests pass, with one previously failing demo-table manifest test explicitly deselected after reproducing it on the original implementation. pdftext 0.6.3 compatibility: 229 tests pass. All 31 documents / 298 pages retain identical complete model-list and Middle JSON output. Sum of five-run document medians: 45.02s → 31.49s (30.1% reduction); no sample exceeds the 5% time/RSS regression threshold. Six reviewed overlay pages are pixel-identical. Wheel build includes all 62 PDF modules; isolated wheel installation produces identical two-page financial-table output.

The original test_demo_sparse_table_confidence_manifest bbox expectation failure is documented separately and its expected data is not modified.

Compatibility

The existing PdfModel.predict(), PDFDocument methods, ModelJson/MiddleJson output and renderer contracts remain unchanged. Recognition thresholds, candidate priority and fallback behavior are preserved.

Checklist

• Relevant Ruff checks and git diff --check pass.
• Focused regression and complete corpus output comparisons recorded.
• New functions have Chinese docstrings and explicit type annotations.
• Documentation or module responsibility docstrings updated.
• Remote CI and CLA checks completed.

Remote check status

license/cla succeeds. The separate CLAAssistant workflow fails because the configured signature branch is missing (Branch cla not found), then requests contributor signatures. Workflow/signature settings are unchanged by this refactor. See https://github.com/myhloli/Magic-PDF/actions/runs/33952244239/job/101269092002 .