feat(experimental): add http.readonly and http.returned field attrs

[ping-maxwell]

Wire and capability edges can reject fields the client must not set, and v.fn output validation strips fields that must not leave in the response. I also lifted the schema walk into core (parseFields / omitFields / rejectFields) so those gates stay attribute-key agnostic.

http.serverOnly still works as a deprecated alias of the readonly gate.

[pkg-pr-new]

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npm i https://pkg.pr.new/better-call@204

commit: 619a07b

[greptile-apps]

Greptile Summary

The PR adds http.readonly and http.returned field attributes and centralizes attribute-based schema projection and rejection.

• Rejects readonly fields at HTTP and capability boundaries while retaining serverOnly as a deprecated alias.
• Projects returned fields out of validated function outputs.
• Adds async-aware union selection and projected fallbacks to the shared schema helpers.

Confidence Score: 4/5

The PR is not yet safe to merge because nested-union defaults and transforms can still execute twice despite the attempted probe-result reuse fix.

The selected outer union arm is validated once by parseFields, but rejectFields subsequently validates nested union arms again, repeating default factories or transform side effects and leaving the previously reported duplicate-validation defect outstanding.

Files Needing Attention: packages/experimental/src/schema.ts

_{Reviews (7): Last reviewed commit: "fix(experimental): reuse union probe res..." | Re-trigger Greptile}