`add_blockref` never checks the name and `add_auto_attribs` skips autofill
when `Insert.block()` resolves to None, so a typo'd block name returned a
handle whose attribute values were silently dropped and whose dangling
INSERT `doc.audit()` later deletes. Check `name in self._doc.blocks` and
raise `ValueError` on both the attribute and no-attribute paths, matching
the COM engine's `InsertBlock` refusal for an unknown name.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd
