doc-drift-scanner.sh has no behavioural test: carry-forward, rollup open/close and ledger are untested #128

Context

scripts/doc-drift-scanner.sh is the only doc-drift component with no behavioural test. CI runs bash -n on it (tests/test-doc-drift-precheck.sh:128) and shellcheck (.github/workflows/tests.yml:43-45), and that is all. The checker and the precheck are covered (tests/test-doc-drift-check.sh, tests/test-doc-drift-precheck.sh) and the fixer's dry-run fail-closed path is covered. The scanner's state machine is not.

Why it matters

The scanner is where a wrong answer is expensive and silent:

• Carry-forward (doc-drift-scanner.sh:131-140): a repo whose checker fails must keep its previous findings and must not have its rollup issue closed. That is the exact class of bug that bit dep-bump before it was caught (tests/test-dep-bump-subset-carryforward.sh, PR feat: Add repoman run orchestrator (v0.1.0 run path) #117).
• Rollup open/close (gh_issue_exists at :215, close_issue_if_valid at :226, --issue-limit): the only code path that writes to other repos.
• Ledger: the shape of latest.json / history.json and the new / fixed / recurring delta.
• --dry-run must not write latest.json (:318 guards the write at :321), otherwise the next live run reports every finding as new.

Proposal

Add tests/test-doc-drift-scanner.sh, driving the real scanner with gh stubbed on PATH — the harness in tests/test-dep-bump-subset-carryforward.sh is the template: stub gh writing to a call log, REPOMAN_CONFIG_FILE / REPOMAN_REPOS_FILE into a temp dir, temp REPORTS_DIR, one real git init clone per enrolled repo, bash 3.2-safe (no mapfile, no associative arrays).

Cases

1. Findings in a repo with no prior issue → creates [doc-drift] owner/name, body lists the corrections, doc-drift label created best-effort.
2. Checker fails for a repo that had findings → previous rows carried into latest.json, issue not closed, run exits 0 and reports failed: 1.
3. All findings gone → the existing issue is closed with the scan id in the close comment.
4. --dry-run → no create/edit/close call and latest.json untouched.
5. --issue-limit 1 → stops after one repo.
6. latest.json carries repos_scanned / corrections / additions / delta, and history.json gains exactly one row.

Touchpoints

• New: tests/test-doc-drift-scanner.sh.
• .github/workflows/tests.yml — both lists are explicit, so an unlisted file never runs in CI: the shellcheck list (:42) and the run list (:62-79).

Notes

Review follow-up from #124 (#pullrequestreview-5742023444).