Motivation

• Remove repeated JWT segment destructuring and formatting to make negative-token generation more DRY and readable.
• Centralize JWT segment/object mutations so future changes to token shape or encoding are localized.

Description

• Introduce a small JwtParts helper (struct JwtParts { header, payload, signature }) with constructors and mutation helpers (generated, with_header_segment, with_payload_segment, with_header_object, with_payload_object, payload_object, into_token) in crates/uselesskey-token/src/srp/shape.rs.
• Replace ad-hoc tuple destructuring/format! uses with JwtParts calls in negative-variant helpers such as malformed_jwt_segment_count, bad_base64url_segment, invalid_jwt_header_shape, missing_alg, mismatched_kid, not_yet_valid_claims, token_with_header_claim, and token_with_payload_claim.
• Keep public behavior unchanged while reducing duplicated code paths and making header/payload manipulation explicit and composable.

Testing

• Ran cargo test -p uselesskey-token to exercise the crate test suite.
• All automated tests for the uselesskey-token crate passed (no failures).

Codex Task