Expand JuliaC trim coverage for JWT verification

[quinnj]

Summary

• Add typed JWT payload decoding for concrete claim schemas with JSON.jl 1.
• Decode standard JWT header claims through a concrete schema during trim compilation.
• Expand strict JuliaC workloads to parse claims and validate signed tokens.
• Keep JSON.jl 0.20 and 0.21 compatibility through the existing dynamic path.

Validation

• Julia 1.12 ordinary suite: 2,664 package checks pass.
• Julia 1.6 with JSON 0.21.4: 2,664 package checks pass.
• Both strict JuliaC workloads compile with zero verifier errors and warnings.
• Both generated executables run successfully.

Scope

Remote JWKS fetching and open-ended dynamic claim dictionaries remain in the ordinary test suite. The strict workload covers local keys and concrete claim schemas.

Co-authored by Codex