feat(contract): implement SEP-41 permit signature support

[mofejo1]

Closes #250

Summary

• add permit function to token contract for off-chain signed allowance approvals
• add permit_nonce getter and per (token_id, owner, spender) nonce tracking
• enforce replay protection by requiring exact nonce match and incrementing nonce on success
• enforce expiry protection using ledger timestamp deadline checks
• emit permit event when permit approvals are applied

Security

• replay attack prevention: nonce must match current stored nonce
• permit expiration: rejects permits after deadline
• no on-chain owner approval transaction needed; relies on Soroban auth entries via require_auth_for_args

Tests added

• test_permit_sets_allowance_and_increments_nonce
• test_permit_expired_panics
• test_permit_replay_panics

Validation note

• cargo test -p sep41-token is currently blocked by a pre-existing workspace manifest error unrelated to this change:
  • src/bridge/Cargo.toml references missing src/lib.rs

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