test: cover the coverage gate's signal-killed exit path

[hivecommons-hive]

Test Improvement

Adds one case to tests/coverage-report-cli.test.mjs covering the exit path
tests/tools/coverage-report.mjs:680 takes when the spawned unit run did not
exit normally.

spawnSync reports a child terminated by a signal as status: null, not as a
number, and process.exit(null) exits 0. Without the ?? 1 fallback the
reporter prints "Tests failed; coverage above is reported for context." and
then reports success — so a run cut short by the OOM killer or a job timeout
would satisfy npm run test:unit:coverage:check on no evidence, leaving only a
stderr line nobody reads on a green job.

The existing sibling case covers a suite that fails and exits 1. That status is
numeric and never reaches the fallback, so deleting ?? 1 leaves the suite
green today.

The new case drives the reporter over a generated suite that SIGKILLs
node --test itself. Three details make it deterministic:

• process.ppid inside a test file is the runner the reporter spawned, one
  level below the reporter, so the suite kills the runner without touching the
  process under test.
• SIGKILL rather than SIGTERM: the runner handles SIGTERM and exits 1, which is
  the numeric path already covered.
• The killed runner flushes no profile and its orphan dies with it, so the
  suite calls v8.takeCoverage() before the kill. Otherwise the reporter stops
  at No coverage data was recorded. and never reaches the path under test —
  which also exits 1, so the assertion excludes it explicitly.

Verification

• node --test tests/coverage-report-cli.test.mjs — 4/4 pass.
• Mutation check: replacing result.status ?? 1 with result.status fails the
  new case and only that case; restoring it passes. Run both ways locally on
  node v26.10.0.
• npm run test:unit:coverage moves tests/tools/coverage-report.mjs from
  99.63% regions (sole uncovered region: line 680) to 100.00% lines /
  100.00% regions.
• npm run test:unit:coverage:check exits 0.
• npx prettier --check tests/coverage-report-cli.test.mjs clean.

Test-only: no file outside tests/ is touched.

Related Issue

Closes #1047

Overlap

Touches tests/coverage-report-cli.test.mjs only. Disjoint from the open
hold-gated PRs: #1040 (tests/tools/e2e-coverage-report.mjs,
tests/tools/e2e-coverage-run.mjs, tests/tools/e2e-coverage-scripts.mjs,
tests/e2e-coverage-report.test.mjs, tests/e2e-coverage-run.test.mjs),
#1034 (tests/e2e-data-fixtures.test.mjs, tests/e2e/data-variants.spec.js),
#1042 (tests/svg-active-content.test.mjs), #1039 and #1044.

— hive: agent=quality backend=copilot model=claude-opus-5 copilot=1.0.88

[hivecommons-hive]

Important

Held for human review by the hive's ACMM level gate.

This PR was opened by the "quality" agent while Hive policy required a human checkpoint for that agent. Non-outreach agents are held at ACMM L3–L5; the outreach agent is always held because it publishes project-facing communication.

Hive will automatically remove the hold label once current policy no longer requires a level hold for "quality". If this is an outreach PR, a human must review it and remove the label.