test(fake-dom): cover the four uncovered regions in the fake document

[hivecommons-hive]

Test Improvement

tests/tools/fake-dom.mjs is the stand-in document every focus-trap test
mounts against, and four of its sub-line regions were unexercised. This adds
tests/fake-dom-harness.test.mjs with one test per region:

• removing a listener for a type that was never registered is a no-op, and
  removing an unregistered handler leaves the registered one in place
• keydownListenerCount() and dispatchKeyDown() behave on a document that
  never saw a listener
• restore() puts back a document that existed before the install
• restore() deletes the global when none existed

The focus-trap suite cannot reach these: it registers exactly one keydown
listener and runs only where no global document exists. The restore arm is
what keeps a fake document from leaking into later tests under a DOM-providing
runner, and nothing proved it worked.

Files touched: tests/fake-dom-harness.test.mjs (new) only. No production
code, no changes to existing tests, so this is disjoint from the open test PRs
(#678, #680, #686, #688, #693, #702, #706), which claim scripts/ fixtures and
tests/{import-architectures,svg-active-content,fetch-community-people,validate-*,collect-*,coverage-report-collect,react-harness}.

Region coverage for tests/tools/fake-dom.mjs: 92.00% -> 100.00%
(TZ=UTC node tests/tools/coverage-report.mjs). Line coverage was already
100%, so this is a pure region gain.

Related Issue

Closes #707

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Superseded by #750, which consolidates this and 14 other test-only coverage PRs into a single reviewable change (commit cherry-picked unmodified, authorship and DCO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