fix(gatsby): Persist manifest on cached builds

[tyhopp]

Description

In our current implementation the client module manifest is wiped on subsequent builds, breaking hydration.

This happens because we run PartialHydrationPlugin during the build-javascript stage, and on subsequent builds webpack doesn't revisit untouched files and uses the cache instead.

This change restores the previous manifest when PartialHydrationPlugin runs and merges it with any new client modules found.

React has a similar implementation looking for .client.js files (the old API) in a beforeCompile hook to iterate over later - https://github.com/facebook/react/blob/3f70e68cea8d2ed0f53d35420105ae20e22ce428/packages/react-server-dom-webpack/src/ReactFlightWebpackPlugin.js#L110-L132. We could do something similar, but it would be more expensive with the new API because we'd need to check every file's contents for client export on every run.

Instead our plugin runs alongside JavaScript bundling, relies on webpack to detect touched files, and adds new client modules to the manifest as needed. This won't handle deletion, but deleted client modules don't need to be referenced from the manifest at runtime anyway since they're not loaded.

The way I see it this change fixes the subsequent builds problem for now, and if we see issues in the future with this approach we can re-evaluate using an approach like React's (and test the performance impact).

Documentation

N/A

Related Issues

[sc-55768]