Have the Router track its deployment block

Prevents a consensus split where some nodes would drop transfers if their node
didn't think the Router was deployed, and some would handle them.

processor/ethereum/router/contracts/Router.sol

@@ -7,6 +7,9 @@ import "Schnorr.sol";
 
 // _ is used as a prefix for internal functions and smart-contract-scoped variables
 contract Router {
+  // The block at which this contract was deployed.
+  uint256 private _deploymentBlock;
+
   // Nonce is incremented for each command executed, preventing replays
   uint256 private _nonce;
 
@@ -63,6 +66,8 @@ contract Router {
   }
 
   constructor(bytes32 initialSeraiKey) _updateSeraiKeyAtEndOfFn(0, initialSeraiKey) {
+    _deploymentBlock = block.number;
+
     // We consumed nonce 0 when setting the initial Serai key
     _nonce = 1;
     // Nonces are incremented by 1 upon account creation, prior to any code execution, per EIP-161
@@ -230,6 +235,10 @@ contract Router {
     return _nonce;
   }
 
+  function deploymentBlock() external view returns (uint256) {
+    return _deploymentBlock;
+  }
+
   function smartContractNonce() external view returns (uint256) {
     return _smartContractNonce;
   }