fix: verify marketplace nft ownership

[dangelo352]

Summary

• verify marketplace list_nft and start_auction sellers own an active NFT via the configured NFT contract
• revalidate existing listing/auction seller ownership before accepting make_offer on those positions
• expose get_nft_contract for visibility and test setup
• add mock-NFT coverage for owned, unowned, inactive, and stale-owner listing/auction/offer paths
• update marketplace lifecycle and security docs

Closes #488.

Validation

• CARGO_TARGET_DIR=/tmp/commitlabs-marketplace-owner-target cargo test -p commitment-marketplace "rejects_" -- --nocapture
• CARGO_TARGET_DIR=/tmp/commitlabs-marketplace-owner-target cargo test -p commitment-marketplace -- --nocapture
• git diff --check

Notes: the marketplace suite passes with existing warnings for an unused seller variable and unused set_contract_reentrancy_guard helper.