TS dev files: unchecked, or published to npm

## What problem does this address? [`tsconfig.base.json` excludes `**/test/**`](https://github.com/WordPress/gutenberg/blob/86802246acef3fc324f311921c14359fd50303bd/tsconfig.base.json#L42): 397 TS test files across 44 packages are never checked: ```sh $ echo "const brokenOnPurpose: number = 'not a number';" >> packages/blob/src/test/index.ts $ npx tsc -b packages/blob --force; echo $? 0 ``` ``` packages/blob/src/test/index.ts(40,11): error TS2304: Cannot find name 'global'. packages/blob/src/test/index.ts(82,31): error TS2339: Property 'toHaveStyle' does not exist on type 'JestMatchers<HTMLAnchorElement>'. ``` Stories are the inverse, type checked but shipped: the `components` tarball carries 248 story `.d.ts` files: ```sh $ cd packages/components && npm pack --dry-run npm notice 672B build-types/alignment-matrix-control/stories/index.story.d.ts npm notice 422B build-types/alignment-matrix-control/stories/index.story.d.ts.map ... ``` Jest types leak into 10+ src projects; `theme` copes with five bespoke tsconfigs. ## What is your proposed solution? Constraints: - All TS files type checked: src, tests, stories, scripts. - No test or story output in `build-types` or tarballs. - Build does not check dev files. - IDE checking works everywhere, correct ambient types. - Jest globals never leak into src (CLI and IDE). - Isolated installs compatible: type deps per package - #75814. - Incremental references, low boilerplate, `lint:tsconfig` enforced, per-package migration. Options: - A: test projects in the root solution; build checks them. - B (preferred): root `tsconfig.json` checks everything; build uses src-only `tsconfig.build.json`; package default configs hold dev files (jest types) and reference build configs. - C: one root project for all dev files; unioned ambient types. - D: strip dev output at publish; rejected, jest leaks. - Orthogonal: emit-only builds via `--noCheck`.