dCDH wrap-up: REGISTRY non-binary Note — close stale negative-baseline deferral

Single CI codex R1 P3 finding: the non-binary `by_path` Note in
REGISTRY still said the negative-baseline regression (`(-1, 0, 0, 0)`)
was deferred, but this PR adds `test_negative_baseline_path_supported`
that covers exactly that regime. Updated the Note to list both
negative-D regression tests (non-baseline and baseline positions) as
shipped Python-only coverage.

No code/test change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber