feat(sources): merge Xcode's Developer Documentation MobileAsset (USR + novel pages)

Xcode 26+ ships the docs corpus as a vector-search SQLite asset
(com.apple.MobileAsset.AppleDeveloperDocumentation). New adapter merges
it into the corpus duplication-free: keyed NULL-guarded backfill of the
symbol USR (new v26 documents.usr column; 212,597 rows on the live
corpus) and per-platform versions, plus insertion of truly-novel pages
only (7,968; anchors and child-granularity rows are skipped — the crawl
stays authoritative). Idempotent; asset opened read-only/immutable.

mobileasset-fetch.js adds the no-Xcode path: resolve the zip URL from
the local MobileAsset manifest (else a pinned fallback), stream-download
from updates.cdn-apple.com (652 MB), verify manifest SHA-1 + size,
extract only documentation-db. Verified live end-to-end. gdmf live
discovery needs an Xcode-private asset audience — left as future work.