fix: Shift field silently ignored — dyadic rational inputs effectively unsupported (#606)

Add require(shift == 0) guards to Optimizer.calculateParams and
OptimizerV3.calculateParams so non-zero shifts revert instead of being
silently discarded.  OptimizerV3Push3 already had this guard.

Update IOptimizer.sol NatSpec to document that shift is reserved for
future use and must be 0 in all current implementations.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

onchain/test/Optimizer.t.sol

@@ -359,6 +359,18 @@ contract OptimizerTest is Test {
         }
     }
 
+    /**
+     * @notice calculateParams reverts when any slot has shift != 0
+     */
+    function testCalculateParamsRevertsOnNonZeroShift() public {
+        for (uint256 k = 0; k < 8; k++) {
+            OptimizerInput[8] memory inputs;
+            inputs[k] = OptimizerInput({ mantissa: 0, shift: 1 });
+            vm.expectRevert("shift not yet supported");
+            optimizer.calculateParams(inputs);
+        }
+    }
+
     /**
      * @notice Non-admin calling upgradeTo should revert with UnauthorizedAccount
      */

onchain/test/OptimizerV3Push3.t.sol

@@ -224,6 +224,17 @@ contract OptimizerV3Push3Test is Test {
         push3.calculateParams(inp);
     }
 
+    // ---- Shift guard ----
+
+    function testNonZeroShiftReverts() public {
+        for (uint256 k = 0; k < 8; k++) {
+            OptimizerInput[8] memory inp;
+            inp[k] = OptimizerInput({mantissa: 0, shift: 1});
+            vm.expectRevert("shift not yet supported");
+            push3.calculateParams(inp);
+        }
+    }
+
     // ---- Fuzz ----
 
     function testFuzzNeverReverts(uint256 percentageStaked, uint256 averageTaxRate) public view {