Add the missing explicit-path regression so operators keep the current fail-fast contract covered, and record the completed ROADMAP follow-up.
Constraint: scope limited to ROADMAP helper regression coverage plus the DONE roadmap entry; tests/__init__.py is included so the requested python -m unittest tests.test_roadmap_helpers invocation resolves locally instead of an external tests package.
Rejected: changing roadmap-next-id.sh behavior | dogfood showed the script already returns the desired nonzero empty-stdout error.
Confidence: high
Scope-risk: narrow
Directive: Keep stdout reserved for the single next id on successful roadmap-next-id.sh runs.
Tested: python -m unittest tests.test_roadmap_helpers -q; scripts/roadmap-check-ids.sh ROADMAP.md; scripts/roadmap-next-id.sh ROADMAP.md; git diff --check -- ROADMAP.md tests/test_roadmap_helpers.py tests/__init__.py
Constraint: roadmap-next-id.sh must preserve single-id stdout on success while failing closed if duplicate validation cannot run.
Rejected: Relying only on CI/pre-push duplicate checks | the helper is used immediately before appending and must not certify an already-poisoned file.
Confidence: high
Scope-risk: narrow
Directive: Keep roadmap-next-id.sh stdout machine-clean; route validation failures and checker availability errors to stderr, and keep focused helper behavior coverage in the docs/ROADMAP CI path.
Tested: scripts/roadmap-next-id.sh ROADMAP.md printed 725 before appending #725 and 726 after; temp ROADMAP with duplicate 999 exited nonzero and listed duplicate id; scripts/roadmap-check-ids.sh ROADMAP.md; bash -n scripts/roadmap-next-id.sh scripts/roadmap-check-ids.sh; python -m unittest discover -s tests -p test_roadmap_helpers.py; python -m pytest tests/test_roadmap_helpers.py -q; SKIP_CLAW_PRE_PUSH_BUILD=1 bash .github/hooks/pre-push
Not-tested: full cargo workspace build, unchanged docs/script-only path
