Multi-chain donor support (ETH mainnet, SOL, AVAX)

[headertag]

SPEC §10 Phase 2.3. Validate reverse-bridge feasibility per chain (open question §9.4).

Acceptance

• Donor source-chain selector wired to anchor route discovery.
• Per-chain refund-address validation (EVM 0x..., Solana base58, AVAX 0x...).
• Confirm reverse-bridge available on each chain before enabling source.