fix: support webpack-dev-server 6.x by dropping SockJS instanceof checks - #1046

…#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/react-refresh-webpack-plugin@0.5.17
```

`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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