fix(vrowzer): give both Workers a default index.html when ready() gets none

[kazupon]

Closes #70

ready() without /index.html, as in the README usage example, now shows a preview: the runtime gives both Workers the same default /index.html.

Cause

The Web Worker wrote a default /index.html during setup. But the Service Worker, which serves the preview HTML from its own virtual filesystem, never got one. A ready() without /index.html therefore left the preview without HTML. Since #69, the preview reported one 404 load error and stayed blank.

The README usage example passes only /main.js, which writes into #app, so following it showed no preview.

Fix

• initialize() adds a default /index.html to the files it copies from ready() when they have none, and sends it to both Workers. An /index.html given to ready() is used as before.
• The default is minimal: an empty <div id="app"> and a module script that loads /main.js.
• The Web Worker no longer writes its own default in setupVirtualFiles(), so the default lives in one place. The Web Worker receives the HTML from the Service Worker, and its default never reached a preview.
• The README section of ready() and the JSDoc of VrowzerConfig.files describe the default. The API references are regenerated. The README examples are unchanged and now work as written.

The Web Worker's old default also had a global CSS reset (* { margin: 0; padding: 0; box-sizing: border-box; }), body padding and a "Loading..." placeholder. They are left out: they would clash with the user's CSS, and the old default never reached a preview.

Behavior change

A ready() without /index.html shows the default page, instead of a blank preview with a 404 load error. A preview always loads /index.html, so no working setup depended on its absence. Deleting /index.html after ready() still leads to a 404, as before.

Not included

• Guessing other entries, such as /src/main.ts. The default loads /main.js.
• Service Worker restarts (Recover project files and channels after Service Worker process restart #46) still lose the files, including the default.
• Instances sharing a Service Worker (Isolate projects sharing a Service Worker registration #45): an instance without /index.html sends the default, which can replace the /index.html of another instance, as any file with the same path already can.

Tests

• Unit tests:

  • index-file-sync.test.ts: a ready() without /index.html sends the same default to both Workers, with #app and /main.js. The existing test that gives an /index.html passes unchanged, so a given /index.html is not replaced. The test from fix(vrowzer): write binary files given to ready() as bytes in both Workers #66 that compares the V_FS_INIT files now expects the default as well.
  • transformer.test.ts: setupWorker() no longer writes /index.html.

  The new checks failed before the change.

• Integration test in the shared playground: opened with ?files=readme-example, the playground gives ready() only the /main.js of the README usage example. The test opens it in a new browser context, which has a Service Worker of its own, and checks that the preview shows "Hello!". Without the default, the preview stays blank and the test times out.

Validated locally with:

• vp run test:unit: 119 files, 1,915 tests
• vp run test:integration:vrowzer: 70 passed and 1 conditional skip, in three runs
• vp run test:e2e: 46 passed, 6 existing test.skip
• oxlint (no new warnings), knip, the format check of the changed files, and typecheck