docs(shadow-prove): clarify version is read for logging only in calc_batch_pi

[chengwenxi]

Summary

• shadow_rollup::calc_batch_pi extracts version from the batch header but never branches on it, which looks inconsistent with the V2/non-V2 split in lib.rs and could be mistaken for a missing dispatch.
• The public-input formula is in fact uniform across versions: offset 57 holds the blob input field (versioned hash for V0/V1, aggregated hash for V2) and the on-chain verifier uses the same layout. Version-dependent routing happens earlier via the batch_version request parameter.
• Add a comment so maintainers don't need to reverse-engineer this from the header layout.
• Addresses audit finding INFO-2 from the 2026-05-15 multi-blob audit (PR feat: multi blob #935).

Test plan

• cargo check -p shadow-proving
• Comment-only change; no behavior modified.

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