test: add vitest unit tests across packages

Cover shared zod schemas, extract traversal/refs/downloads, and tts filename/error handling. Document the test setup in the README and docs.

docs/architecture.md

@@ -89,6 +89,7 @@ packages/extract ──► packages/tts (traverseMod) ──► packages/shared
 ## Tooling dependencies
 
 - `typescript` (strict), `tsx` (dev runner), `eslint`, `prettier`.
+- `vitest` (unit tests), colocated as `*.test.ts` next to sources.
 - `pnpm` workspaces for package management.
 
 ## Deployment / runtime shape

docs/implementation-plan.md

@@ -192,7 +192,8 @@ packages/extract    →  flatten objects / extract refs / download assets
 
 - TypeScript strict mode.
 - `tsx` for dev, `tsc` for build.
-- Root scripts: `pnpm dev`, `pnpm build`, `pnpm lint`.
+- `vitest` for unit tests, colocated as `*.test.ts` next to sources.
+- Root scripts: `pnpm dev`, `pnpm build`, `pnpm test`, `pnpm lint`.
 
 ## Build order