bridge: design + security plan (epic)

[TheNewAutonomy]

Goal

Define bridging approach and security assumptions for moving assets/messages between Catalyst and external chains.

Scope

• Finality model assumptions and confirmation depth
• Proof model (receipts + inclusion proofs + state roots)
• Threat model for bridge relayers/oracles
• Phased plan: testnet bridge -> hardened bridge

Acceptance criteria

• Written design doc with clear security model
• Implementation plan broken into follow-up issues