feat(plugin-vite): enable main process hot restart via exported API

[erickzhao]

Continues @bglgwyng's work in #4210, which could not be pushed to directly since it lives on a fork. Their commits are preserved here; the fixes from a review pass are added on top.

Closes #4210 (superseded).

What this adds

plugin-vite gains an opt-in hotRestart option. With it enabled, rebuilding the main process bundle during electron-forge start restarts the running app automatically, rather than requiring rs in the terminal.

new VitePlugin({
  build: [/* ... */],
  renderer: [/* ... */],
  hotRestart: true,
})

The mechanism is a small internal API in core-utils: start() installs a restart handler, and the Vite plugin's closeBundle hook requests a restart through it. In dev, vite.build() runs in-process, so the two share a module singleton. It has no effect when packaging, where builds run in a subprocess that cannot reach the app.

Fixes on top of #4210

Correctness:

• Only ignore an app exit when that child was restarted. The CLI checked the first child it ever spawned, whose restarted flag stays true forever, so every exit after the first restart was swallowed along with its exit code.
• Queue a restart requested while one is already in flight. lastSpawned is briefly null between kill and respawn, so a rebuild landing in that window was reported as "nothing to restart" and dropped.
• Don't let a late close discard a replacement child that has already been installed.
• Don't restart on a failed build. Rollup passes the build error to closeBundle before rethrowing it, so the app was restarted onto a stale bundle and silently ran the previous build's code.
• Report a failed relaunch instead of rejecting unobserved, which would take down the Forge process.
• Initialize ElectronProcess.restarted, which is declared non-optional but was left undefined until the first restart.
• Catch a throwing restart handler. Callers are bundler hooks, so a throw surfaced a restart failure to the user as a build failure.

Design:

• Replaced the module-level EventEmitter with a single handler slot plus a disposer. Two handlers would race to kill and respawn the same child process.
• Exposed the restart API under a @electron-forge/core-utils/restart subpath so plugin-vite and the packaging subprocess don't pull in the whole barrel (@electron/rebuild, find-up, semver, …), and kept it out of the public entrypoint since it's internal.
• Gave the two plugin instances distinct names (:hot-restart / :hot-reload) rather than sharing one.
• The warning for a rebuild that fails to reach the app lives in the Vite plugin, not in requestAppRestart. Only the plugin can distinguish a first build — where the app isn't spawned yet and a no-op is correct — from a rebuild.

Testing

19 new tests covering the restart handler slot, the plugin's closeBundle behavior, and the restart lifecycle in start(). Full fast project passes (429 tests); yarn build and yarn lint are clean.

Notes for review

• hotRestart defaults to false and is not enabled in the vite / vite-typescript templates. Worth deciding whether it should be.
• The restart handler is a process-wide singleton, so two overlapping start() calls contend for it and only the most recent app stays restartable. Threading the capability through the postStart hook instead would be cleaner, but that's a larger redesign than this branch should carry.
• Merged next to resolve conflicts. next had since replaced chalk with node:util's styleText, so this branch's new output was migrated to match, and the rs handler's inline restart logic on next was dropped in favour of the extracted restartRunningApp().

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