Pickles: fix standard proof serialization for chunked proofs.

Two-chunk proofs now preserve all `prev_evals.evals.public_input` chunks when round-tripped through `Proof.to_base64` / `Proof.of_base64`. This makes the normal o1js JSON proof path work for chunked proofs without requiring a separate chunked serialization helper.

Adds regression coverage for a two-chunk proof with public output, asserting that deserialized proofs keep both public-input evaluation chunks and still verify.
