feat: soc pubkey

[nugaon]

Checklist

• I have read the coding guide.
• My change requires a documentation update, and I have done it.
• I have added tests to cover my changes.
• I have filled out the description and linked the related issues.

Description

soc.FromChunk now retains the owner's secp256k1 public key, exposed via a new OwnerPubKey() getter. recoverAddress returns the recovered public key (compressed) alongside the owner address, and FromChunk stores it on the SOC struct.

Motivation and Context (Optional)

FromChunk already runs EC public key recovery (crypto.Recover) to derive the owner address, but discarded the recovered public key. Recovery is expensive, so persisting the public key lets later operations reuse it without recomputation — e.g. Diffie-Hellman key exchange (crypto.DH.SharedKey) using the owner key obtained from a SOC.

Related Issue (Optional)

#5435
ethersphere/SWIPs#93

Screenshots (if appropriate):

AI Disclosure

• This PR contains code that has been generated by an LLM.
• I have reviewed the AI generated code thoroughly.
• I possess the technical expertise to responsibly review the code generated in this PR.