Function write_document 

pub fn write_document(
    path: &Path,
    doc: &Document,
    docs: &BTreeMap<PathBuf, Document>,
    dir: &Path,
) -> Result<()>

Write one Document under dir — the real, typed write boundary (P7.6, #1309, Decision C: one place derives the sibling-path relationship instead of two independently-maintained ones). Shared by write_output and bynk-driver::test_runner’s own output loop (bynkc test), so every disk-writing path emits maps uniformly (slice 2 — bynkc test --inspect runs the emitted .ts directly and needs the maps on disk).

A Ts document is printed through the one printer that owns a character (R7.3); its own Printed::source_map is discarded, not written — the real map, when one exists, is already present in docs as its own SourceMap entry at this path’s .map sibling (sibling_path), split out at construction (bynk-emit::project::build_output‘s own tail). That sibling’s presence, not Printed::source_map, is what decides whether the .ts/.js file gets a //# sourceMappingURL= trailer — the trailer lives only on the on-disk artefact, not on docs’ own in-memory text, so golden comparisons are unaffected.