Document project-creation gap in cascading settings

What does this MR do and why?

Documents the project-creation gap in cascading settings: cascade workers fire on parent updates, not child creation, so new projects get the database default instead of inheriting the parent value. Adds guidance on the EE::Projects::CreateService fixup pattern that duo_features_enabled and ai_audit_events_storage_enabled already use.

This gap is undocumented and has caused bugs (the auto_duo_code_review_enabled incident, !200397 (merged)) and recurring review confusion (!242583 (merged), !255428 (merged)).

References

• !200397 (merged) (the auto_duo_code_review_enabled bug that required a 43M-row BBM)
• !209323 (merged) (write-cascade conversion for auto_duo_code_review_enabled)
• !242583 (merged), !255428 (merged) (review discussions about this pattern)

Screenshots or screen recordings

No UI changes.

How to set up and validate locally

1. Read the updated doc/development/cascading_settings.md and confirm the new "Handle new project creation" subsection is accurate.

MR acceptance checklist

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