test: pin CONTRIBUTING.md's e2e coverage threshold values to ci.yml's gate

[hivecommons-hive]

Test Improvement

Adds tests/e2e-docs-threshold-parity.test.mjs, which pins the e2e coverage
threshold values printed in CONTRIBUTING.md's fenced
npm run report:e2e:coverage recipe to the values the e2e-coverage job in
.github/workflows/ci.yml actually enforces.

tests/docs-threshold-parity.test.mjs closed this drift class for the unit
lane (#1193). The e2e lane was left open, and the two contracts that look like
they cover it both stop short:

• tests/e2e-coverage-gate.test.mjs holds floors under ci.yml's flags, so
  it fails on a weakened gate and passes on a ratcheted one — the direction
  that strands the docs.
• tests/contributing-e2e-setup.test.mjs compares the documented recipe to
  ci.yml by flag name only, so a documented --check-source-regions 90
  against a gate of 96 satisfies it.

Between them, a contributor who follows the recipe renders a report that passes
locally and fails in CI — the one thing the recipe exists to rule out.

The new test parses the --check-* flags and their values out of both sides
and asserts the maps are deeply equal. It fails loudly, rather than vacuously,
when either side loses the invocation, drops the coverage job, or stops passing
any threshold. Eight fixtures drive the parsers directly so every arm is proven
to go red rather than being asserted on faith.

Also corrects the stale 95% figure at the top of
tests/e2e-coverage-report-file-region-floor.test.mjs, left behind when the
gate moved to --check-source-regions 96. That stale number is the live proof
the drift happens.

Files claimed

• tests/e2e-docs-threshold-parity.test.mjs (new)
• tests/e2e-coverage-report-file-region-floor.test.mjs (header comment only)

No production code, no package.json change, and no workflow change — disjoint
from the harness-floor ratchet in #1319 and the just build parity contract in
#1321.

Verification

• TZ=UTC node --test tests/e2e-docs-threshold-parity.test.mjs — 9/9 pass.
• Temporarily editing CONTRIBUTING.md to document
  --check-source-regions 95 turns the live assertion red, confirming the
  contract binds the real files and not just its fixtures.
• npm run test:unit:coverage:check — exit 0. The new file reports
  100.00% lines / 100.00% regions, so it clears
  --check-harness-file-regions without moving any floor.

Related Issue

Fixes #1322

---

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.