test(validators): cover the seven uncovered fallback-label regions

[hivecommons-hive]

Test Improvement

Line coverage is already 100% for every source file in this repository, so the
remaining unit-test gaps are all sub-line regions. This PR closes one
coherent cluster of seven of them: the fallback labels the validator scripts
attach to an error when the record that caused it has no natural identifier,
plus the two || [] guards that keep a collector output missing its array
from turning a validation failure into an iteration TypeError.

Regions covered (file / line / region):

File	Line	Region
scripts/validate-architectures.mjs	51	|| '<unknown>'
scripts/validate-architectures.mjs	71	|| '<unknown>'
scripts/validate-case-studies.mjs	60	?? entry.slug
scripts/validate-case-studies.mjs	60	?? 'unknown'
scripts/validate-launch-metrics.mjs	87	|| []
scripts/validate-launch-metrics.mjs	97	|| '(missing id)'
scripts/validate-metrics.mjs	80	|| []

Tests are appended to the four existing validator test files and use the
established runScriptWithFixtures sandbox; no production code changes, no
new test helpers, no new files.

The assertions pin the observable behaviour rather than the coverage number:
each test checks the exact [error] <label>: <message> line a maintainer
would read, and asserts the label is not undefined — and for the two
|| [] guards, that stderr carries the real validation failure rather than a
TypeError.

Verification

TZ=UTC node --test tests/validate-architectures.test.mjs \
  tests/validate-case-studies.test.mjs \
  tests/validate-launch-metrics.test.mjs \
  tests/validate-metrics.test.mjs     # 96 pass, 0 fail
TZ=UTC npm run test:unit              # 1186 pass, 0 fail, 2 todo
npx prettier --check tests/           # clean

Region coverage for the four files, measured before and after with
tests/tools/coverage-report.mjs:

File	Before	After
scripts/validate-architectures.mjs	94.59%	100%
scripts/validate-case-studies.mjs	93.75%	100%
scripts/validate-launch-metrics.mjs	93.33%	100%
scripts/validate-metrics.mjs	97.96%	100%

Scope

Files touched: tests/validate-architectures.test.mjs,
tests/validate-case-studies.test.mjs,
tests/validate-launch-metrics.test.mjs, tests/validate-metrics.test.mjs.

Deliberately disjoint from the open hold-gated PRs: #678 (import-architectures
regions), #680 (svg-active-content regions), #686 (fetch-community-people
regions), #674 (the region-coverage gate in package.json), #684
(validate-projects-born, validators-smoke). No file, script, or region in
this PR is claimed by any of them.

Related Issue

Closes #687

---

Filed by quality agent (hold-gated mode). Human review required.

— 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.

[mrbobbytables]

Superseded by #750, which consolidates this and 14 other test-only coverage PRs into a single reviewable change (commit cherry-picked unmodified, authorship and DCO preserved).