Existing direct DOC underline

DocDocument.paragraphs[index].runs[index].directUnderline reads and edits an existing direct CHPX sprmCKul operand. Supported values are none, single, and double (Kul bytes 0, 1, and 3). Absence, style inheritance, piece PRMs, duplicate operands, opaque context and other underline styles return undefined or refuse mutation. No formatting record is inserted and styles or underline colors are not resolved.

const paragraph = document.paragraphs[1];
paragraph.runs[0].directUnderline = 'double';
const bytes = document.serialize();

Writes reuse the exclusive full physical-run ownership checks. Shared CHPX blobs, partial paragraph views, overlapping logical pieces including other stories, PAPX aliases and embedded controls refuse. A supported edit changes only the existing one-byte operand; unrelated stream and formatting bytes remain unchanged. Invalid primitive values throw UnsupportedOle2EditError without coercion or document mutation. No-ops keep a clean document clean. Run handles expire after a successful edit; obtain a fresh handle from the paragraph for the next edit.

The official MS-DOC Character Properties defines sprmCKul, and Kul defines its enumeration. This API reports direct stored exceptions rather than resolved appearance or full document fidelity.

prepare-native-doc-underline-edits.mjs produces reproducible edits of the repository-owned fixture after a build. It does not launch Office or claim native validation itself. Independent native Word validation must compare per-character underline, all other captured fonts, paragraph formats, text, styles, stories and document counts, including save/reopen.