feat: gate batch escrow bids with blinded signer

[ozwaldorf]

Summary

• Replace the batch escrow bond deposit with an EIP-712 BatchBondAuth signature gate; bidding is now free for nodes
• A bid's signature must recover to the escrow's blindedSigner and binds both the bidding EOA and the exact claimed transfer rows
• Drop all deposit collection, the bondAmount field, and the forfeited-bond reward rollover
• Add ECDSA recovery helpers, a ZeroBlindedSigner constructor guard, and the BatchBondAuth test helper
• No bond pot yet: nodes are paid only the pro-rata reward share on collect (deferred until economics are settled)

Details

• Signature scope binds (bondingExecutor, uint256[] transferIndexes) so one enclave authorization is single-purpose and cannot be replayed across a different row subset in the same escrow
• Multi-bidder model retained: any EOA with a valid enclave signature can commit a disjoint subset concurrently
• Off-chain follow-up: nomad's contracts.rs needs a BatchBondAuth(address bondingExecutor,uint256[] transferIndexes) typed-data path alongside the existing BondAuth; domain (MirageEscrow/1) is unchanged
• Regenerated artifacts/batch_*.hex for the CI bytecode guard; updated README
• 110 tests pass, including both real-Tempo-block end-to-end proofs

[g4titanx]

@ozwaldorf what's the update on this