fix: add marketplace emergency pause

[dangelo352]

Summary

• add admin-only pause/unpause/is_paused controls to commitment_marketplace
• gate list_nft, buy_nft, accept_offer, place_bid, and end_auction while paused
• keep read-only marketplace getters callable during pause
• document marketplace incident-response pause/unpause in emergency/security docs

Closes #491.

Validation

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

Notes: the marketplace test run passes with the repo's existing warnings for an unused seller variable and unused set_contract_reentrancy_guard helper.