Fix synopsis extraction: read the Sphinx definition line, not the signature head

Extractive synopsis: lead every api chunk with the page's own summary sentence

Cut oversized sections at natural seams; embed only the new parts

Codebase quality pass: kill silent-truncation hacks, fix latent bugs, cover them (#46)

Fix polluted anchors/titles, purge stale chunks, add channel debug to search

M2 ingestion: discover (objects.inv + sitemap), crawl (hash-diff snapshot), heading chunker, CI