docs(prover): explain why header.state_root replaces morph_diskRoot

[chengwenxi]

Summary

• The host executor was migrated from the custom morph_diskRoot RPC field to the standard block.header.state_root in commit e0756a4, but the rationale was not captured in the code.
• After the zkTrie → MPT migration (PR Feat: Upgrade Prover to SP1 v5, Integrate Morph-Reth as STF, and Replace zkTrie with MPT #886) the header state_root is the authoritative MPT root, so the custom field became redundant.
• Add a short comment at both post_state_root and prev_state_root so future readers don't need to dig through git history to understand the equivalence.
• Addresses audit finding INFO-1 from the 2026-05-15 multi-blob audit (PR feat: multi blob #935).

Test plan

• cargo check -p prover-executor-host
• Comment-only change; no behavior modified.

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