[a5768549]

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Summarize your changes:

Fixes #4439.

During electron-forge start, the preload watch build asked the parent to send a full-reload from closeBundle on every build, including the first one. The first build finishes before Electron is spawned, so no client is connected yet. Vite buffers a full-reload that finds no client and delivers it to the first client that connects, so the window's page reloads right after it first loads.

subprocess-worker.ts now only forwards reload-renderers for rebuilds, the same way pluginHotRestart uses builtOnce.

Tests

• Added a subprocess-worker spec that runs a preload target in watch mode, copied to a temp dir, and records the IPC messages: the first build sends only first-build-done, and appending to the preload entry then sends reload-renderers. It fails on main (the first build also sends reload-renderers) and passes with this change.
• yarn vitest run --project fast packages/plugin/vite/spec/ passes, except getBuildDefine:serve with custom server host in vite.base.config.spec.ts, which fails the same way on an unmodified main on my machine (Windows 11, Node 24).
• oxfmt --check and oxlint are clean for packages/plugin/vite.

The fix for #4446 is separate and sent as its own PR. Both add a test to subprocess-worker.spec.ts, so whichever lands second may need a trivial rebase there.