feat: implement canonical nonce strategy across contracts

[Cedarich]

• Implements a canonical nonce model (per-caller sequential nonce + used-nonce log) via remitwise_common::nonce and applies it consistently across remittance_split , savings_goals , and orchestrator .
• Updates orchestrator entrypoints to use consistent nonce read → execute → increment semantics and adds get_nonce for callers.
• Adds/updates integration tests covering nonce advancement, replay rejection, wrong nonce rejection, and savings snapshot replay protection.
• Updates THREAT_MODEL.md to document the canonical nonce strategy and security assumptions.
• Fixes a duplicate proptest dev-dependency entry in insurance/Cargo.toml

closes #340

[Baskarayelu]

@Cedarich Please resolve the conflicts

[Cedarich]

@Baskarayelu On it boss 🙏🏿

[Cedarich]

@Baskarayelu Conflicts resolved