Raw output pointers (diff-ref mode)

The generated patch is committed; per the diff-ref decision rule the verbatim content is the diff itself:

git diff 3a5cfde0~1..3a5cfde0 -- docs/_ext/d2diagrams.py

Generation notes (non-code, verbatim)

RenderResult enum added; render_svg() return type changed bool -> RenderResult. Mapping in run(): FAILED -> state_machine.reporter.error(...), NO_OUTPUT -> raw .d2 source as a literal_block, OK/FELL_BACK -> image.
Atomicity via tempfile.mkstemp(dir=out.parent, ...) + os.replace(tmp, out); temp unlinked on any failure path.
Collision guard: module-level _seen_outputs: dict[str,str] keyed by output basename, cleared by a builder-inited handler connected in setup().
parallel_read_safe/parallel_write_safe kept True but now justified by the atomic swap (documented inline).
Empirical verification performed before commit:
- broken .d2 + sphinx-build -W -> exit 1, ERROR node rendered, committed actor_tree.svg md5 unchanged,
- duplicate-stem orphan page -> “d2 output collision” error,
- nix run nixpkgs#d2 forced re-render -> deterministic (no git drift), no .tmp residue,
- ruff check clean.