feat(poc): add terrors PoC crate scaffold and error types

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

protobufs/client.proto

@@ -23,6 +23,7 @@ message ClientRequest {
   oneof payload {
     AuthChallengeRequest auth_challenge_request = 1;
     AuthChallengeSolution auth_challenge_solution = 2;
+    arbiter.evm.EvmSignTransactionRequest evm_sign_transaction = 3;
   }
 }