commit pi as hash digest

[hero78119]

This PR migrates public input/output handling from raw public-io memory openings to a digest-based path and unifies PI plumbing across prover/verifier/recursion.

Design rationales

• Avoids carrying vector-style public-io MLE openings through proving/verifying paths.
• Reduces PI plumbing complexity and duplicated logic between native verifier and recursion verifier.
• Makes public output commitment explicit and uniform via digest commitments.

Changes Highlights

• Commit public IO as a Keccak-256 digest (8 x u32 words) instead of opening raw public-io MLE columns.
• Add runtime commit APIs in ceno_rt:
  • commit(data: &[u8])
  • commit_digest([u32; 8])
  • CommitCtx for incremental accumulation + finalize.
• Add PUB_IO_COMMIT syscall support end-to-end:
  • emulator syscall implementation in ceno_emul/src/syscalls/pubio_commit.rs
  • zkVM ecall instruction in ceno_zkvm/src/instructions/riscv/ecall/pubio_commit.rs
  • RV32IM wiring and dispatch updates.
• Introduce digest limb layout/query path:
  • PUBIO_DIGEST_IDX, PUBIO_DIGEST_U16_LIMBS
  • query_public_io_digest() and PubioCommitLayout.
• Simplify PI representation:
  • ProofInput.pi replaces pub_io_evals/public-input MLE plumbing
  • remove pi_in_evals, instance_openings, raw_pi_num_variables, and related split/eval paths.
• Update recursion verifier/binding to consume scalar PI directly and keep transcript order aligned with native verifier.
• Remove legacy public-io memory region usage from platform/linker init flow (no preloaded pubio segment).