chore(vitest): add coverage reporting

[ubugeeei]

Problem

`vitest run` exits 0 even when a file has zero coverage. We have no signal whether security helpers or animation math are tested at all.

Expected

• Enable v8 / istanbul coverage in `vite.studio.config.ts` (`test.coverage`).
• Add `npm run test:coverage` (alias of `vitest run --coverage`).
• Document the floor in `docs/docs/production.md`.

Acceptance

• `npm run test:coverage` produces a text report.
• CI artifact (optional) uploads coverage summary.