test(e2e-coverage): pin attribution to converter.branches and drop the dead functions arm

[hivecommons-hive]

Test Improvement

convertScript (tests/tools/e2e-coverage-report.mjs) built attributedPaths
from two arms, each behind a ?? {} guard. Three of those regions can never be
evaluated:

• the converter.functions arm. v8-to-istanbul writes this.functions[path]
  only under a truthy block.functionName (lib/v8-to-istanbul.js:167,178), and
  normalizeRanges rebuilds every script's V8 record as exactly one block with
  functionName: ''. converter.functions is therefore permanently {}.
• both ?? {} guards. this.branches = {} and this.functions = {} are set
  in the converter's constructor (lib/v8-to-istanbul.js:36-37) and never
  reassigned.

This reduces the set to the arm that actually decides attribution and records
why the others are gone, so they are not restored later as defensiveness.

Removing a guard is only safe while the premise under it holds, and nothing in
this repository held v8-to-istanbul — a pinned, Dependabot-bumped dependency —
to it. So the same change adds tests/e2e-coverage-attribution.test.mjs, which
states the premises against the installed converter rather than its source,
plus the end-to-end consequence:

1. branches and functions are {} after load() and object-valued after
   applyCoverage, so neither needs a nullish guard;
2. an anonymous block attributes both sources of a two-source map through
   branches and none through functions;
3. a named block does populate functions, for its first range's source only
   — non-vacuity for (2), and the reason that arm would be the weaker signal
   even if normalizeRanges ever named a block;
4. a two-source bundle keeps both sources in report.sources, including one
   whose coverage is a single uninterrupted covered range — the case
   collectE2ECoverage:786-791 would drop silently, with no diagnostic, if
   attribution ever stopped naming it.

No production code and no workflow changes: both files are under tests/.

Verification

At 4184f33, clean clone, npm ci, node v26.10.0, TZ=UTC:

• node --test tests/e2e-coverage-attribution.test.mjs → 4 tests, 0 failures.
• npm run test:unit:coverage:check → 2321 tests, 0 failures, exit 0.
• Harness region coverage 99.32% → 99.53% (1471/1481 → 1467/1474);
  tests/tools/e2e-coverage-report.mjs 97.98% → 98.57%, its uncovered
  region lines going from 244 627 686 689 691 782 793 1017 1096 to
  244 627 796 807 1031 1110 — the 686/689/691 trio, and nothing else.
• npx prettier --check clean on both files.

Scope and overlap

Claims exactly one cluster: the attributedPaths expression in convertScript
(tests/tools/e2e-coverage-report.mjs) and the new
tests/e2e-coverage-attribution.test.mjs. Disjoint from #1303, which is on
splitRangeAtBoundaries and its line-627 [range] fallback in the same file,
and from #1294 (tests/tools/coverage-report.mjs, tests/helpers*.mjs scoring)
and #1298 (threshold floors in package.json / tests/coverage-gate-thresholds.test.mjs).

Related Issue

Closes #1304

---

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 keep the hold label until a human removes it. Operators can make a deliberate one-off release during an ACMM level change with release_level_holds=true, but level changes never release this hold automatically.