Add Vitest coverage reporting so we can see how much of the codebase the tests exercise.

🧠 Context

The project uses Vitest (pnpm test → vitest run). Coverage is not enabled. Vitest has built-in coverage (the equivalent of pytest-cov) via a provider package.

The goal here is reporting only — print a coverage summary and generate an HTML report. Do not add an enforced threshold that fails the build. The test suite is still small, so a hard gate would just block PRs; a threshold can be added later once coverage is meaningful.

🛠️ Implementation Plan

1. Install the coverage provider: pnpm add -D @vitest/coverage-v8 (the default V8 provider — fast, native). This repo has security settings in pnpm-workspace.yaml; if the install is blocked by a policy error, flag it to Jacc rather than working around it.
2. Add a test:coverage script to package.json: vitest run --coverage.
3. Configure coverage in vite.config.ts under test.coverage:
   • provider: 'v8'
   • reporter: ['text', 'html'] (terminal summary + an HTML report)
   • sensible excludes: src/main.tsx, **/*.test.ts, config files (*.config.*), type-only files (src/types/**), and the test setup.
   • No thresholds.
4. Run pnpm test:coverage and confirm it prints a summary table and writes the HTML report.
5. Add the coverage output directory (e.g. coverage/) to .gitignore. (This also keeps it out of prettier/eslint, which respect .gitignore.)
6. Before pushing, run pnpm format then pnpm typecheck, pnpm lint, pnpm format:check, and pnpm test — CI now gates on all of these, so your vite.config.ts / package.json edits must be formatted.

🚫 Out of scope (stretch / later)

• Wiring coverage into CI (.github/workflows/ci.yml) and uploading an lcov artifact — a reasonable stretch goal, but keep the first PR to local reporting unless you want to take it on.
• Enforced coverage thresholds — stick to report-only for now. We don't want CI immediately breaking.

✅ Acceptance Criteria

• pnpm test:coverage runs and prints a coverage summary in the terminal
• @vitest/coverage-v8 is added as a devDependency
• test.coverage is configured in vite.config.ts with sensible excludes
• No enforced threshold (reporting only)
• The coverage output directory is gitignored
• pnpm typecheck, pnpm lint, pnpm format:check, and pnpm test all pass