Make test coverage measurable

[sepehr-safari]

Good First Issue

Coverage cannot currently be measured, even though the project states a target for
it.

vitest.config.ts configures coverage with provider: 'v8' and reporters, and
CONTRIBUTING.md line 189 states "Coverage target: 70%+ for core package". But
@vitest/coverage-v8 is not a dependency and there is no script to run it, so the
command fails outright:

$ npx vitest run --coverage
MISSING DEPENDENCY  Cannot find dependency '@vitest/coverage-v8'

That means the stated 70% target has never been checked and nobody can tell whether
it is met.

What to do

1. Add @vitest/coverage-v8 to the root devDependencies. Match the installed
   vitest version, which is 3.2.7, so ^3.2.7.
2. Add a script to the root package.json:

   "test:coverage": "vitest run --coverage"

3. Run pnpm test:coverage and confirm it produces a report.
4. In the pull request description, paste the summary table and say whether the
   core package clears the 70% line. Do not change any thresholds or add
   coverage.thresholds in this pull request; reporting the real number is the
   whole point, and what to do about it is a separate decision.

Files to modify

• package.json (devDependency and script)
• pnpm-lock.yaml (pnpm install rewrites this, commit the result)

No changeset: this changes tooling only, and nothing in the published package
changes.

Guidelines

• Run the full local check from
  Verify Locally
  before pushing.
• Do not add the coverage run to .github/workflows/ci.yml in this pull request.
  Whether CI should gate on coverage is a separate decision.
• Branch off current main.

How to claim

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