fix(header-footer): use section-aware page numbering for odd/even parity (SD-2991)

[luccas-harbour]

Summary

• Switches odd/even header/footer parity selection from the physical page index to a new section-aware displayNumber so headers respect OOXML <w:pgNumType w:start="…"> and per-section number restarts (ECMA-376 §17.10.1).
• Threads a displayNumber field through the layout pipeline (Page → HeaderFooterPage → ResolvedPage → ResolvedHeaderFooterPage) and stamps it from activePageCounter inside layoutDocument.
• Adds a parityPageNumber option to the headerFooterUtils selectors (getHeaderFooterType, getHeaderFooterTypeForSection, getHeaderFooterIdForPage, resolveHeaderFooterForPage(AndSection)) so callers can override parity when needed. Also fixes the legacy single-section selector to use % 2 !== 0 for odd, so negative display numbers route to odd instead of falling through.
• HeaderFooterSessionManager now reads displayNumber when picking header/footer variants for both per-rId layouts and inferred region variants.

Why

When a section sets <w:pgNumType w:start="2"/> (or otherwise restarts numbering), Word treats the first body page as page 2 and renders the even header on it. We were keying odd/even off the physical page index instead, so the wrong variant was selected — and downstream space reservation (header content height) used the wrong height, shifting body content vertically.

Changes by package

• layout-engine/contracts: add optional displayNumber on Page, HeaderFooterPage, ResolvedPage, ResolvedHeaderFooterPage.
• layout-engine/layout-engine: rename documentPageNumber → parityPageNumber in getVariantTypeForPage, feed it activePageCounter, and stamp state.page.displayNumber during page creation.
• layout-engine/layout-bridge: new parityPageNumber option threaded through every header/footer selector; existing callers default to page.displayNumber ?? page.number. Negative-odd parity now resolves to 'odd'.
• layout-engine/layout-resolved: forward displayNumber from layout to resolved page/header-footer outputs.
• super-editor/HeaderFooterSessionManager: use displayNumber for both region variant inference and per-rId variant lookup; preserve it when forwarding pages into header/footer layout requests.

Tests

• layout-engine: new uses section page-numbering start for odd/even header parity test (section with numbering.start = 2 swaps body offsets on pages 1 and 2). Existing alternateHeaders tests updated to describe behavior in terms of display number.
• layout-bridge/headerFooterUtils: new tests cover the parityPageNumber override, negative-odd handling, section-aware display-number parity, and caller-supplied parity overrides.
• layout-resolved/resolveHeaderFooter: asserts displayNumber is preserved through resolution.
• super-editor/HeaderFooterSessionManager: new tests verify displayNumber parity is used both when selecting per-rId header layouts and when inferring region variants.

[linear]

SD-2991

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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