Handle ready() without /index.html, as in the README examples #70

Current implementation

Reviewed at cdac4cd.

• The README usage example and the example in the ready() section pass no /index.html.
• The Web Worker's setupVirtualFiles() writes a default /index.html with <div id="app"><p>Loading...</p></div> and <script type="module" src="/main.js"></script>, then writes the files given to ready(). It was added in 72ee3f0, a WIP commit with no stated reason.
• The Service Worker's initServiceWorker() writes only the client files and /public/.gitkeep. The files given to ready() arrive later in V_FS_INIT (index.ts). No default /index.html reaches it.
• The Service Worker serves the preview HTML from its own virtual filesystem. indexHtmlMiddleware moves on when the file is missing, and notFoundMiddleware answers with 404.
• Every test fixture passes /index.html: the integration fixtures (playground, custom-base, dispose) explicitly, and the e2e fixtures through the vite-plugin manifest or explicitly. No test runs ready() without /index.html. The test added in #69 reproduces a missing preview HTML by loading /favicon.ico instead.

Verification

I ran temporary tests in packages/vrowzer/integration/vrowzer.integration-test.ts against the integration playground, in Playwright's Chromium. The tests deleted /index.html with deleteFile(), then mounted a new preview session. This leaves the Service Worker in the same state as a ready() without /index.html. The tests were not committed.

• At cdac4cd (with fix(vite-dev-server): answer missing files within the preview base path with 404 #69): the session reported one previewLoadError with stage: 'html', status: 404 and the message Failed to load the preview HTML: 404, and its body stayed empty. Nothing more was reported within 4.5 seconds.
• At fce16a3 (before fix(vite-dev-server): answer missing files within the preview base path with 404 #69): the preview HTML request got the 503 "Waiting for Service Worker..." page of @vrowzer/vite-plugin. Within 4.5 seconds, the iframe loaded 5 times and reported 5 previewLoadError events.

Options

• A. Give the Service Worker the same default.
  • In initialize(), add the default /index.html to the files sent to both Workers when ready() has none. The default then lives in one place.
  • Or write the same default in service-worker-core.ts too. The default then lives in two places.
• B. Require /index.html.
  • Remove the Web Worker's default, and add /index.html to the README examples.
  • Make ready() warn, or fail, without /index.html.
• C. Only add /index.html to the README examples, and keep the Web Worker's default as it is.

The choice depends on whether a default index.html is part of the API. The README examples are currently written as if it were.

Acceptance criteria

• Both Workers agree on /index.html when ready() gets none: both have the same default, or neither has one.
• The README examples work as written.
• The behavior without /index.html is documented: a default page is shown, or the missing file is reported clearly.
• A browser test covers ready() without /index.html.