chore: release 0.5.1

[maish]

Cuts 0.5.1 with two stitching correctness fixes.

Fixed

• Numeric singleton rows were folded into the preceding row during multi-page stitching (merger.py). Sparse numeric rows such as financial subtotals and fair-value-only lines are now preserved as independent rows; text-only wrapped-cell continuations still fold as before.
• Headerless fragments could duplicate their first numeric row during Docling injection (adapters/docling.py). When extraction demotes an upstream reader's erroneous column_header flag, injection now emits that row exactly once as ordinary data instead of preserving it as a header and repeating it in the body.

Notes

• Adds demoted_numeric_header to TableMeta and LogicalTable — additive, defaulted False, so no caller breaks.
• Output shape changes for affected tables: the orphan-pair integration fixture goes 101 → 103 rows, which is the fix itself.
• Also refreshes the stale CHANGELOG link refs at the bottom ([Unreleased] still compared against v0.3.0; 0.4.0/0.5.0 refs were missing).

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