test: cover the e2e reporter's source-reference resolution branches

[hivecommons-hive]

Test Improvement

Covers five reachable branches in the reference-resolution path of
tests/tools/e2e-coverage-report.mjs — the containment gate that decides which
source files e2e coverage is attributed to. Test-only; no tooling change.

line	branch	new test
98	rel === '' in repoRelative	sourcePathFromReference rejects a reference that resolves to the repository root
109	slash === -1 ? '' in sourcePathFromReference	sourcePathFromReference rejects a webpack reference with no path after the authority
199, 202, 234	the three sourcesContent fallbacks in normalizeSourceMap	collectE2ECoverage reads original content from disk when the map omits sourcesContent

Why these were worth pinning

• 98 — isInside explicitly accepts the root directory itself (rel === ''
  at line 93). The only thing that then rejects the repository root as a
  source file is repoRelative returning null for an empty relative path.
  That arm had nothing holding it.
• 109 — webpack://<authority> with no following slash must yield an empty
  path. Had it regressed to returning the unsliced value, the reference would
  have resolved against the map's own directory inside build/ rather than
  being rejected.
• 199 / 202 / 234 — sourcesContent is optional in the source-map spec, but
  all 28 existing fixtures in this file supply it, so the entire
  no-sourcesContent path was unexercised. The new fixture declares both a
  src/ source and a node_modules/ source and omits sourcesContent
  entirely, so one assertion covers reading the src source from disk and
  blanking the external one.

Measured effect

npm run test:unit:coverage at 900592b, node v26.10.0, TZ=UTC:

	regions	uncovered lines in e2e-coverage-report.mjs
before	90.68%	98 109 188 199 202 234 255-256 301 303 318-319 322 325-327 329 331 405 408 410 424 485 495 497 509 520 549 565 708 751
after	92.23%	188 255-256 301 303 318-319 322 325-327 329 331 405 408 410 424 485 495 497 509 520 549 565 708 751

Exactly the five targeted regions left the list; nothing else moved. Repository
total 94.87% → 94.94% against the --check-regions 94 floor.

Line 188 (map.resolvedSources ?? flattened.sources ?? []) is deliberately left
alone — AnyMap always populates resolvedSources, so it appears unreachable
and is not claimed here.

Verification

• node --test tests/e2e-coverage-report.test.mjs — 38 passed, 0 failed.
• npm run test:unit:coverage:check — exit 0.
• npx prettier --check tests/e2e-coverage-report.test.mjs — exit 0.

Scope note

This is a unit branch-coverage improvement in test tooling. It makes no
claim about any source path's end-to-end coverage: Playwright could not be
launched in this environment (chrome-headless-shell fails on a missing
libglib-2.0.so.0; installing it needs root), and a published e2e-coverage
artifact cannot be rendered without the build that produced it — the subject of
the already-open #1040.

Coordination

Appends to tests/e2e-coverage-report.test.mjs only; no .mjs tooling file is
touched, so there is no logic overlap with any open PR.

• test: make the e2e coverage artifact renderable without its build #1040 edits convertScript and the script-root resolution inside
  collectE2ECoverage.
• test: fold out phantom e2e coverage regions over covered lines #1051 edits getRegionCoverage and the region union.
• test: name the run in e2e coverage tooling-error artifacts #1070 edits main()'s catch.

None of them touch sourcePathFromReference, repoRelative or
normalizeSourceMap. All of them also append to this test file, so the only
possible interaction is an append-order rebase.

Related Issue

Closes #1071

— 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.