Why: downstream parsers are unreachable via extension.Host and the base lacked xlsx/pptx parsers; MS1 needs Excel and PPT ingest (EXT-5 request, documents proposal 2026-10-06-ext-5-host-parse-document).
Evidence: local GOWORK=off go test ./... all 29 packages pass incl. new ooxml_test (hand-built parts), ooxml_python_test (openpyxl/python-pptx round-trip), inproc owner-scope and limit tests, xlsx upload e2e; go vet clean; recorded as SELF on the Dev task card, PR CI to confirm.
Impact: Host gains ParseDocument (pure compute; errors only on ctx cancel or >100MiB input); builtin:xlsx (sheet order via workbook.xml rels, rich-text shared strings, formula cache, 1900/1904 dates, sparse cells) and builtin:pptx (sldIdLst order, titles, group shapes, AlternateContent Choice, tables, notes) added with 200k-cell/64MiB-part caps; legacy doc/xls/ppt get an actionable hint; ParsersDescription gains xlsx/pptx and the missing text ext; surface snapshot now records exported interface methods; docx part reads share the 64MiB cap.
Refs: documents 2026-10-06-ext-5-host-parse-document.md; EXT-1 host.go precedent.