tax rate, version and compose (#70)

resolves #67

Co-authored-by: johba <johba@harb.eth>
Reviewed-on: https://codeberg.org/johba/harb/pulls/70

onchain/src/Kraiken.sol

@@ -16,8 +16,18 @@ import { Math } from "@openzeppelin/utils/math/Math.sol";
  */
 contract Kraiken is ERC20, ERC20Permit {
     using Math for uint256;
-    // Minimum fraction of the total supply required for staking to prevent fragmentation of staking positions
 
+    /**
+     * @notice Protocol version for data structure compatibility.
+     * Increment when making breaking changes to TAX_RATES, events, or core data structures.
+     * Indexers and frontends validate against this to ensure sync.
+     *
+     * Version History:
+     * - v1: Initial deployment with 30-tier TAX_RATES
+     */
+    uint256 public constant VERSION = 1;
+
+    // Minimum fraction of the total supply required for staking to prevent fragmentation of staking positions
     uint256 private constant MIN_STAKE_FRACTION = 3000;
     // Address of the liquidity manager
     address private liquidityManager;