docs(audit): document hash-chain tamper evidence

[maxw06]

Closes #1703.

Summary

• Add quicklendx-contracts/docs/AUDIT_HASH_CHAIN.md documenting the audit hash-chain design.
• Cover genesis hash semantics, per-entry linking, hash preimage serialization order, and operation tag mapping.
• Document verifier/divergence usage, edge cases, and detected versus out-of-scope tampering.
• Link the guide from the contract README.

Validation

git diff --check origin/main..HEAD
rg -n "fn (entry_hash|last_entry_hash|verify_audit_chain|first_audit_chain_divergence|genesis_prev_hash)|operation_tag|append_" quicklendx-contracts/src/audit.rs quicklendx-contracts/docs/AUDIT_HASH_CHAIN.md quicklendx-contracts/README.md

[Baskarayelu]

thanks for the contribution! our workflow only merges a PR when its author is assigned to the linked issue — please get assigned to an open issue first, then raise the PR. closing as invalid for now 🙏