Add zh-CN RustChain market framing preamble

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Summary

• Add a zh-CN market framing section for the existing Chinese README
• Position RustChain around China second-hand hardware, repair, and reuse culture
• Explain Proof of Antiquity as a hardware age proof / machine time fingerprint, with anti-VM-farm and anti-Sybil context
• Keep the copy non-hype and include operating-cost / no-guaranteed-return caveats

Bounty: Scottcjn/rustchain-bounties#12445

Validation

• git diff --check

RTC wallet: will provide on merge

Disclosure: drafted with AI assistance and manually reviewed before submission.