feat(header): add CAPSULE_PROTOCOL standard header (RFC 9297)

[paroga]

• The description, docs, and comments (words meant for humans) are written by a human, not by an LLM. (https://seanmonstar.com/micro/20260729-i-want-your-own-words/)

[seanmonstar]

Thanks for contributing!

However, I think this header is infrequent enough that we don't need to have it as a built-in variant. It's possible for user code to make their own const CAPSULE: HeaderName = HeaderName::from_static("capsule");.