Align canonicalization and wire helpers with spec review decisions

[jt55401]

Implement the protocol decisions from the spec review.

Tasks:

• Use canonical unpadded Base64 for hashes and signatures across helpers and tests.
• Align claims canonicalization with the spec decision that all direct child meta elements inside signed-section are signed claims.
• Add or update helpers for full origin binding where this repo builds signature payloads.
• Add canonicalization support or test coverage for the initial signed attribute allowlist: href, src, alt, and aria-label.
• Align endorsement verification with the structured endorsement format selected in the draft.
• Add conformance fixtures for claims, signed attributes, origin, encoding, nested sections, and server-HTML snapshot inputs.

Spec tracking issue: HTMLTrust/htmltrust-spec protocol cleanup after security review.