docs(security): document emergency-withdraw timelock and nonce model

[designartvis]

Closes #1725

Description

Documents the emergency-withdraw timelock, expiration window, nonce cancellation model, rejection conditions, and operator runbook for QuickLendX contracts.

Type of Change

• Documentation update
• Security enhancement

Changes Made

Files Modified

• quicklendx-contracts/README.md

New Files Added

• quicklendx-contracts/docs/EMERGENCY_WITHDRAW.md

Key Changes

• documented the initiate -> unlock -> execute / expire / cancel state machine
• documented when execute rejects for missing, cancelled, locked, expired, and insufficient-surplus states
• documented the single pending slot, monotonic nonce, cancellation marker, and public helper entrypoints
• added an operator runbook using get_pending_emergency_withdraw, emg_time_until_unlock, emg_time_until_expire, and can_exec_emergency

Testing

• No breaking changes introduced
• Documentation updated if needed
• No sensitive data exposed
• Unit tests pass
• Integration tests pass

Test Coverage

• Ran: git diff --check -- quicklendx-contracts/README.md quicklendx-contracts/docs/EMERGENCY_WITHDRAW.md
• Not run: cargo is unavailable in this local environment (cargo command not found)

Related Issues

Closes #1725

Additional Notes

This is a docs-only change. Function names and rejection paths were checked against src/lib.rs, src/emergency.rs, src/errors.rs, and src/events.rs.