When the dominant heading level repeats and becomes the split backbone,
a long section is sub-split via SplitText and every sub-chunk inherited
the section-level breadcrumb. Deep `###`/`####` headings that define a
sub-chunk's meaning were dropped from Chunk.ContextHeader, so queries
about content nested far below a shallow heading failed to retrieve it.
Track the heading hierarchy across the section and assign each sub-chunk
the breadcrumb in effect at its start offset, so the deepest active
heading reaches the breadcrumb. Adds a regression test.
Fixes#1674
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
adawang
8d3030d339