Refactor public IO commit flow: add digest-based `ceno_rt::commit` + `PUB_IO_COMMIT` precompile

[hero78119]

Problem

Today ceno_rt::commit compares a guest value against preloaded public IO memory (pubio_read_slice) and asserts equality in guest runtime. This ties commit semantics to a public memory region and a dedicated public-io circuit path.

Current behavior is centered around:

• ceno_rt/src/mmio.rs (commit<T>, pubio_read_slice)
• Public IO initialization in host/e2e paths (ceno_host, ceno_zkvm/src/e2e.rs)
• PublicValues.public_io: Vec<u32> in ceno_zkvm/src/scheme.rs

We want to move to digest-based commit semantics handled by syscall + precompile, and remove the old public IO memory/circuit flow.

Proposal

1) Rewrite guest API: ceno_rt::commit

• Change ceno_rt::commit signature to accept raw bytes:
  • pub fn commit(data: &[u8])
• Internally:
  • Compute Keccak digest (via tiny_keccak)
  • Convert digest into [u32; 16], where each element stores one u16 limb in the low 16 bits
• Send digest limbs through a new syscall (PUB_IO_COMMIT-style syscall)

2) Add syscall + precompile path

• Introduce new syscall for public-io commit digest submission
• Implement new precompile: pubio_commit
• Assume syscall payload is always Keccak digest limbs ([u32; 16] with 16-bit limbs)

3) Extend public values

• Update PublicValues to include extra digest field:
  • e.g. pubio_digest: [u32; 16]
• Wire this through proving/verification/public input queries as needed

4) Update host/CLI/e2e input preparation

• In ceno_zkvm/src/e2e.rs and ceno_cli, prepare public io from args by:
  • serializing bytes (same format as guest input expectation)
  • computing Keccak digest
  • converting to [u32; 16] limbs
• Pass digest limbs into PublicValues instead of raw public IO memory vector semantics

5) Remove old public IO circuit/memory path

• Remove original public-io-memory-related circuit plumbing now replaced by precompile
• Remove dependencies on loading/verifying public IO via dedicated memory range where no longer required

Scope / Affected Areas

• Runtime guest API:
  • ceno_rt/src/mmio.rs
  • ceno_rt/src/lib.rs exports
• Syscall/ecall handling:
  • new syscall opcode/plumbing
  • instruction wiring near existing ecall paths (see ceno_zkvm/src/instructions/riscv/ecall/halt.rs for style/context)
• Precompile:
  • add pubio_commit module and registration
• Public values and PI query:
  • ceno_zkvm/src/scheme.rs (PublicValues, constructor, index/query, MLE usage if needed)
• Execution pipeline:
  • ceno_zkvm/src/e2e.rs
  • ceno_cli public io arg handling (ceno_cli/src/commands/common_args/ceno.rs)
• Cleanup:
  • remove/deprecate old public-io-specific circuit/memory plumbing

Implementation Checklist

• Change ceno_rt::commit API to commit(&[u8])
• Add Keccak hashing + digest-to-[u32;16] limb conversion in runtime
• Define and wire new PUB_IO_COMMIT syscall
• Implement/register pubio_commit precompile
• Add digest field to PublicValues and update constructors/consumers
• Update e2e.rs to compute and pass digest limbs
• Update ceno-cli to hash public io args and produce digest limbs
• Remove old public io memory-based circuit path
• Update guest examples (e.g. examples/examples/fibonacci.rs) to new commit usage

Acceptance Criteria

• Guest programs can call ceno_rt::commit(&[u8]) without reading from public-io memory
• Prover/verifier agree on public commitment via digest limbs ([u32;16], low-16-bit populated)
• PublicValues includes commitment digest and verification uses it
• CLI/e2e public input path produces identical digest/limbs to runtime rules
• Old public io memory circuit path is removed and no longer required
• Existing e2e flows pass with updated commitment mechanism

[hero78119]

Done via #1277