wp-build: Document rolling deployment safety

[MaggieCabrera]

What?

Add focused guidance for generated PHP loaders in the wp-build package, based on Gutenberg 23.2.0.

Why?

During rolling deployments, a generated entry point can become available before one of the files it loads. Generated PHP should treat that window as a temporary missing artifact instead of causing a fatal error.

Evidence:

• Fix: Guard require_once calls in generated PHP files against deployment race conditions #78110 — the deployment-race fix authored by @mikachan and reviewed by @scruffian.

Discussion

This is a release-learning proposal, and the new package-level instruction is intentionally open for review. Please challenge the conclusion, scope, or placement if it would not consistently help future build work. It is also fine to close this PR if the guidance is not useful—the discussion here will help us tune the release-knowledge skill before it becomes part of the release process.

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Co-authored-by: MaggieCabrera <onemaggie@git.wordpress.org>

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[scruffian]

I feel like this is pretty general guidance for any require_once calls - it's not something specific to Gutenberg.