EPBDS-16211 E0: wire up TypeScript, zero behaviour change

[AlexSamBY]

§9.6's whole track was blocked on this: an imported .ts would not build today, so nobody could convert even one file. No product code changes.

babel.config.js	@babel/preset-typescript, listed last so it runs first — presets apply in reverse, and types must be stripped before preset-env/preset-react parse the file
tsconfig.json	new, checker-only: strict, noEmit, allowJs + checkJs: false, isolatedModules, experimentalDecorators, jsx: react
npm run typecheck	tsc --noEmit, a CI step in verify beside the linters
lint:js	--ext now includes .ts,.tsx

checkJs: false is what makes the migration incremental: unconverted JS resolves but is not checked, so a .ts file may import any of the ~330 .js files without demanding they be converted first — while every converted file is strict from its first day.

Babel 7, deliberately

@babel/preset-typescript@8 peer-requires @babel/core@^8 and this project is on core 7, so the current default install fails with ERESOLVE. Pinned to ^7 rather than reaching for --legacy-peer-deps — that flag would install a Babel-8 preset against a Babel-7 core, papering over a real mismatch instead of resolving it.

Two things the probe found, which is what it is for

.test.ts had no Jest globals typed. The first TypeScript test in E1 would have broken the gate on day one. @types/jest added, matched to the installed major — types 30 ↔ Jest 30.3.0; npm's default ^29 would have been a major behind.

lint:js ran --ext .js,.jsx, so every .ts file would have slipped past the zero-warning gate. The ESLint config already listed src/**/*.ts in its overrides and eslint-config-react-app already ships @typescript-eslint — only the CLI flag excluded them. Side effect: src/library/types/index.ts and UIRender.tsx are linted for the first time (0 problems).

The probe is now a permanent guard

Item 4 asked for a probe, run and discarded. Discarding it leaves no .ts under include — so the gate would be green over zero TypeScript, which looks exactly like green over working TypeScript, right up until someone tries to convert something.

src/toolchain/typescriptSupport.ts + its test stay instead. Both pipelines read them: tsc proves the checker sees .ts, Jest proves Babel strips it. Verified to fail when the preset is removed, and to pass when restored. It sits outside src/core/src/library so it is outside collectCoverageFrom, nothing imports it, and it is confirmed absent from dist/index.js. Its own comment says to delete it once real converted modules cover the same ground.

This is the one place I went beyond the letter of E0 — say the word and I will drop it back to a throwaway.

Measured

typecheck	1.6 s over 375 source files (329 .js, 11 .jsx, 35 .json)
non-vacuous	a deliberately mistyped file makes it exit non-zero; green again when removed
suite	166 → 167 suites, 2527 → 2529 tests — exactly the guard
coverage	97.04 / 94.06 / 96.88 / 97.76, unchanged; thresholds green
CSS	static/all.css sha256 unchanged, dcdb0a40a6d8ee79…

The throwaway probe, wired into src/library/index.js, passed typecheck, Jest, build-lib and the demo build — and its output string was found in dist/index.js, proving TypeScript really compiles into the published bundle rather than being silently skipped.

A correction to the plan's own text

E0 said "all four pipelines". There are three — library build, demo build, Jest. watch-lib stopped being a fourth when webpack.watch.config.mjs was deleted under H7. Recorded in the plan.

Next gate is the plan's own: go/no-go after E1, on measured conversion velocity, before committing to the E2/E3 tail.

🤖 Generated with Claude Code