feat: don't merge tables separated by a section heading

[phyohan18]

Problem

Tables that share a column schema but belong to different sections were being stitched into one logical table, because header similarity alone drove the merge. Real-world trigger: an insurance policy's eight per-benefit plan grids — each a Prestige | Elite | Classic table under its own 38a … 38h heading — collapsed into 2 merged blobs, attached to the wrong headings.

Insight

A genuine page-split continuation has nothing but page furniture (running headers/footers) between its fragments. So a section heading sitting between two tables in reading order is a reliable "these are separate tables" signal.

Change

• New MultiPageConfig.block_on_intervening_content (default True) and TableMeta.content_before.
• The docling adapter computes content_before per table in body reading order.
• Both merge paths consult it (symmetric guard): _classify_sequential_pair (pass 1) and should_force_orphan_merge (pass 2).

Avoiding regressions on legitimate continuations

• Only section_header/title block. Paragraphs, list items, captions, footnotes and figures are ignored — real PDFs scatter those between fragments of a single continued table (interleaved reading order, cell text extracted as body nodes).
• Running headers are detected by near-identical recurrence (Jaccard ≥ 0.8) across pages, so a repeated banner — or a journal name docling labels page_header on one page and section_header on the next — does not block a continuation.

Tests

• 5 merger-level guard tests (test_merger.py).
• 3 adapter-level tests building real DoclingDocuments (test_intervening_content_guard.py).
• Full suite: 158 passed, 2 skipped. The existing continuation fixtures (repeated-header, headerless-continuation, orphan-pair, inconsistent-header-detection) are unaffected.

Known limitations

• Two genuinely distinct same-schema tables separated only by a paragraph with no heading still merge. The guard only ever reduces merges, so it cannot introduce new wrong merges — but it doesn't catch that case.
• A real heading that legitimately repeats near-identically on ≥2 pages (e.g. two chapters both titled "Notes") is treated as furniture. Rare; block_on_intervening_content=False is the escape hatch.

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