fix: debounce fs route hmr events, avoid reload for existing routes

[katywings]

What is the current behavior?

• When starting the server, multiple reloads and invalidations are triggered per added route
• When renaming routes, multiple reloads and invalidations are triggered (file rename = "remove file + add file")
• When just changing an existing route, it still triggers a full client reload

Screencast_20260704_131753.mp4

What is the new behavior?

• Reloads and invalidations are debounced
• Changing an existing valid route triggers a soft HMR, no client reload
• Adding a default export to an existing invalid route file, adds the route and reloads the client
• Removing a default export from an existing valid route file, removes the route and reloads the client

Screencast_20260704_131925.mp4

Other information

From what I can tell the route HMR has been broken atleast since #2125. I tried to make sure that I didn't introduce a regression. The routes graph is still updated when adding new files.

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Latest commit: 3f28b00

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@solidjs/start	Patch

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