build: vitest follow-ups — drop test compilation, prune deps, pin versions

[edmundmiller]

Stacked on #246 (migrate-vitest). Base branch is migrate-vitest.

Follow-up improvements to the AVA→Vitest migration:

1. Stop compiling tests via tsc. Drop test/** from tsconfig.json include so npm run build no longer emits lib/test/* (removes the double-collection footgun). Added rootDir: "." to keep lib/src/** output stable for main and the ncc package step. New tsconfig.test.json (noEmit) still typechecks tests, wired into build as tsc && tsc -p tsconfig.test.json, so CI keeps typechecking tests with no workflow change.
2. Prune unused devDeps. Removed ts-node and tsx (unreferenced).
3. Exact-pin vitest deps. vitest and @vitest/coverage-v8 → 4.1.10 to match the repo's Renovate exact-pin convention.
4. Console coverage summary. Added "text" to vitest.config.ts coverage reporters (lcov path unchanged, Codecov step untouched).

Also updated eslint.config.mjs: switched typed-lint from projectService: true to an explicit project array so test files still resolve after the include change, and gave *.config.ts a typed parser project.