always generate 404.html

[Rich-Harris]

• closes invalid static requests return index.html instead of 404 #9011
• closes feat: add fallback option to adapter-cloudflare #9245 (which this PR builds on)

If the user creates their own 404 route or static asset, that will be given priority.

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[Rich-Harris]

thanks @ajgeiss0702!

[danawoodman]

fwiw it appears this PR has broken adapter-cloudflare for anyone using something like an auth hook which shouldn't run during build: #9386