feat(problem): a problem missing a param its message needs is sent as its status's problem

[giraffesyo]

Summary

A type can declare Params its localized message uses, but nothing made a handler pass them. Without one, a client formatting the message gets use-intl's fallback, the raw key (apiErrors.name_taken), instead of a sentence.

Now the problem itself guarantees it:

• Type.New and At/AtParameter remember the type's declared params.
• Problem.MarshalJSON sends a problem missing one as the about:blank problem for its status (keeping status, detail and instance). FieldError.MarshalJSON sends a field error missing one as the invalid rule, keeping its location. This covers problem.Write, huma's serializer, and anything else that encodes a problem.
• (*Problem).Resolve() returns a problem as it will be sent, for callers that build another representation from it (core's legacy error envelope).
• README documents it, and drops a leftover "document the codes" step from before fix(problem)!: adding a problem code no longer changes an API's OpenAPI types #6.

Testing

• make check
• TestMissingParamIsSentAsStatusProblem and TestFieldMissingParamIsSentAsInvalid decode the sent JSON with and without the param.

Checklist

• make check passes
• A new or changed shared message is in every language in packages/problem/src/messages/ (none changed)
• No code, rule or param was renamed (they are API contract)