Migrate jest→vitest, capture coverage baseline.

[dlrice]

Purpose

Establish a documented coverage baseline by migrating the test runner from jest to vitest and recording what the inherited test suite covers. This is the starting line subsequent testing work (including the SSI/RSMF grant work) will be measured against. No new tests, no source refactor.

Approach

• Swap jest → vitest: update package.json scripts and devDeps, add vitest.config.ts (jsdom + v8 coverage), remove .babelrc.
• Add import { describe, test, expect } from 'vitest' to the three inherited specs (globals is off). No other test changes.
• Document commands, CI, and the coverage baseline in a new README Testing section.
• Updated packages.

Testing

No new tests — this PR measures the existing suite, it doesn't expand it.

• yarn test (lint + types + unit) passes; 29/29 tests across 3 spec files.
• yarn test:coverage runs cleanly; numbers recorded in the README.

Visual changes

None.

Checklist

• My PR is scoped properly, and "does one thing only"
• I have reviewed my own code
• I have checked that linting checks pass and type safety is respected
• I have checked that tests pass and coverage has at least improved, and if not explained the reasons why
• If needed, the changes have been previewed by all interested parties.