EPBDS-16211 G: compile JSX with the automatic runtime - #129

…#130)

## The problem, introduced by #128

#128 moved `webpack-dev-server` to 6.
`@pmmmwh/react-refresh-webpack-plugin` 0.5.17 declares it as an optional
peer only up to 5.x:

```
peerOptional webpack-dev-server@"3.x || 4.x || 5.x" from @pmmmwh/[email protected]
```

`npm ci` installs the lock regardless, which is why #128's and #129's CI
stayed green. But every `npm install <package>` since then aborts with
`ERESOLVE`. It was found while adding a devDependency for the React 19
leg.

## The fix

`@pmmmwh/react-refresh-webpack-plugin` `^0.5.16` → `^0.6.3`. 0.6.3
(2026-08-27) adds `webpack-dev-server` 6.x to that peer range
([#1046](pmmmwh/react-refresh-webpack-plugin#1046)
upstream). The one breaking release between them, 0.6.0, raised minimums
we already exceed (Node 18.12, webpack 5.2, webpack-dev-server 4.8). It
also removed `overlay.sock*` options we never set: the demo uses `new
ReactRefreshWebpackPlugin()` with no options.

## Checked

- **The install that failed now succeeds**, run as `npm install
--dry-run --save-dev <the same packages>`.
- **`npm ls --all` reports no invalid peer**, and `npm audit` stays at
0.
- **Fast refresh works on the new plugin.** In the running demo (`npm
start`), the refresh runtime is injected (`__reactRefreshInjected`). A
temporary edit to `NavTabs.jsx`, reverted after, is hot-applied: `[HMR]
Updated modules: ./src/demo/components/NavTabs.jsx` and `[HMR] App is up
to date.` A marker set on `window` beforehand survives, so the page did
not reload.
- **The published package is unchanged.** All 282 files `build-lib`
publishes are byte-identical to master's; the plugin is demo-only.

## Gates

All exited 0:
- `lint:js`, `lint:css`, `typecheck`;
- `test:env-flags`, `test:coverage`, `build`, `build-lib`;
- `test:react16` and `test:react17`, 2907 tests on each;
- the manifest contract.

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The peers now read `^16.14.0 || ^17.0.0 || ^18.0.0 || ^19.0.0`, widened
additively, so hosts on 16.14, 17 and 18 change nothing. §8's fast path is
complete: gates 1 and 3 were met, gate 2 lapsed with the dependency trim, and
this is gate 4.

- The suite. React 19.3.0 is a fixture leg like the 16.14 and 17 ones: an
  install-only package (scripts/fixtures/react19) that plain `npm ci` nests,
  mapped in at resolve time, and `npm run test:react19` runs it locally.
  The harness self-checks the loaded version in every worker, as before.
  React 19 has no legacy root and ships react-dom/client, so this leg renders
  through createRoot: floors.js records `legacyRoot: false`, which skips the
  RTL patch, the ReactDOM.render assertion and the react-dom/client stub.
- CI. `react-19` replaces the `continue-on-error` `react-19-advisory` job,
  which installed 19 over the lock with --legacy-peer-deps. The new job
  needs neither.
- The packed artifact. `test:pack:peers` now server-renders it on 16.14.0,
  17.0.2 and 19.3.0; `test:pack:consumer` covers 18.3.1.
- The types. `test:types` compiles the published declarations against
  @types/react 19 as well, both import styles; 19 is the version that
  removed the global JSX namespace. gen-ts stays on the dev tree's 18.

README, the demo's docs and changelog, CLAUDE.md and the plan record it. The
changelog also records #129's `react/jsx-runtime` import, which a host that
aliases `react` can see.

Local gates exit 0: the React 18, 16, 17 and 19 legs pass 2907 tests each.
Two first runs lost a worker to SIGSEGV (the Node 24 V8 crash,
nodejs/node#62393): test:react19 once and test:react17 once. Both passed
2907/2907 on rerun.

On master since #130 merged, which fixed the `npm install` peer conflict
this change needed gone to add a devDependency. The rebased tree is
identical to the one the local gates below ran on.

## The flip

`peerDependencies`: `react` and `react-dom` now read **`^16.14.0 ||
^17.0.0 || ^18.0.0 || ^19.0.0`**. The range is widened additively, so
hosts on 16.14, 17 and 18 change nothing. This closes §8's fast path:
- gates 1 (Semantic UI React gone) and 3 (`defaultProps`, then the
automatic JSX runtime in #129) were met;
- gate 2 (the form stack) lapsed when `dependencies` was emptied (#128
records why);
- this PR is gate 4.

## How 19 is gated, in three matrices

| | 16.14 | 17 | 18 | 19 |
|---|---|---|---|---|
| whole jest suite | `test:react16` | `test:react17` | `npx jest` /
`test:coverage` | **`test:react19` (new)** |
| packed artifact server-rendered | `test:pack:peers` |
`test:pack:peers` | `test:pack:consumer` | **`test:pack:peers` (added
19.3.0)** |
| published `.d.ts` compiled | `test:types` | `test:types` |
`test:types` | **`test:types` (added `@types/react` 19.3.0)** |

- **The suite on React 19 is a fixture leg, like 16.14 and 17.**
`scripts/fixtures/react19` is an install-only package pinning
`react`/`react-dom` 19.3.0. Plain `npm ci` nests it, it is mapped in at
resolve time, and `npm run test:react19` runs it locally too. The
harness still asserts the loaded React (19.3.0) and scheduler (0.28) in
every worker.
- **What differs for 19.** React 19 has no legacy root and ships
`react-dom/client`, so the leg renders through `createRoot` as the
default suite does. `floors.js` records `legacyRoot: false`, which skips
three things that exist only for 16/17: the RTL `legacyRoot` patch, the
`ReactDOM.render` assertion and the `react-dom/client` stub.
- **CI.** The new `react-19` job replaces the `continue-on-error`
`react-19-advisory` job, which installed React 19 over the lock with
`--legacy-peer-deps`; the new job needs neither. No required checks are
configured on master today, so `react-19` gates exactly as
`react-16-floor` and `react-17` do. If branch protection ever named
`react-19-advisory`, it should name `react-19` now.
- **Types.** The published declarations compile against `@types/react`
19, with both the interop default import and the direct CommonJS import.
That matters because 19 removed the global `JSX` namespace. `gen-ts`
stays on the dev tree's 18; the plan's "pin `@types/react@19` for
`gen-ts`" item was written assuming the dev tree would move to 19, and
it does not.

## Docs

- README and the demo's docs page state the new range.
- The demo's changelog (Unreleased) says React 19 is supported and how
it is tested. It also records #129's change: the bundle imports
`react/jsx-runtime` from the host's React, which a host that aliases
`react` to one copy has to alias too. Two older Unreleased paragraphs
that said the range was not widened yet are updated, so the section does
not contradict itself.
- `CLAUDE.md` lists the three React legs.
- The plan closes §8's gate 4, the CI-coverage row, the `gen-ts` row,
the Appendix C "React 19 flip" checklist and §10's Phase 7 React 19
item.

## Gates

All exited 0:
- `lint:js`, `lint:css`, `typecheck`, `typecheck:contract`;
- `css:fixture:check`, `docs:props:check`, `docs:views:check`;
- `test:env-flags`, `test:types`, `test:coverage`, `build`;
- `test:pack`, `test:pack:peers` (16.14.0, 17.0.2, 19.3.0);
- `test:react16`, `test:react17`, `test:react19`, 2907 tests on each, as
on 18;
- `test:e2e`, 41 Playwright tests;
- the manifest contract.

Two first runs lost a jest worker to `SIGSEGV`, each killing one suite
before its tests ran: `test:react19` once and `test:react17` once. That
is the Node 24 V8 garbage-collector crash
([nodejs/node#62393](nodejs/node#62393)),
fixed on `v24.x-staging` but not in a release yet. Both passed 2907/2907
on rerun. CI runs Linux, where it has not been seen.

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