fix: address review — add negative-mantissa guard to OptimizerV3, add OptimizerV3 test file

- Add require(mantissa >= 0) to OptimizerV3.calculateParams validation loop
  (was missing unlike Optimizer and OptimizerV3Push3)
- Create onchain/test/OptimizerV3.t.sol with shift, negative-mantissa, and
  basic bear/bull output tests
- Fix stale comment in Optimizer.sol: "shift=0 assumed" → "shift=0 enforced"
- Use implementation-neutral NatSpec phrasing in IOptimizer.sol

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

onchain/test/OptimizerV3.t.sol

@@ -0,0 +1,85 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+pragma solidity ^0.8.19;
+
+import {OptimizerV3} from "../src/OptimizerV3.sol";
+import {OptimizerInput} from "../src/IOptimizer.sol";
+import "forge-std/Test.sol";
+
+/**
+ * @title OptimizerV3Test
+ * @notice Unit tests for OptimizerV3.calculateParams input validation guards
+ *         (shift and negative-mantissa) and basic bear/bull output correctness.
+ */
+contract OptimizerV3Test is Test {
+    OptimizerV3 v3;
+
+    function setUp() public {
+        v3 = new OptimizerV3();
+    }
+
+    // ---- Helpers ----
+
+    function _inputs(uint256 percentageStaked, uint256 averageTaxRate)
+        internal
+        pure
+        returns (OptimizerInput[8] memory inp)
+    {
+        inp[0] = OptimizerInput({mantissa: int256(percentageStaked), shift: 0});
+        inp[1] = OptimizerInput({mantissa: int256(averageTaxRate), shift: 0});
+    }
+
+    // ---- 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");
+            v3.calculateParams(inp);
+        }
+    }
+
+    // ---- Negative mantissa guard ----
+
+    function testNegativeMantissaSlot0Reverts() public {
+        OptimizerInput[8] memory inp;
+        inp[0] = OptimizerInput({mantissa: -1, shift: 0});
+        vm.expectRevert("negative mantissa");
+        v3.calculateParams(inp);
+    }
+
+    function testNegativeMantissaSlot1Reverts() public {
+        OptimizerInput[8] memory inp;
+        inp[0] = OptimizerInput({mantissa: int256(95e16), shift: 0});
+        inp[1] = OptimizerInput({mantissa: -1, shift: 0});
+        vm.expectRevert("negative mantissa");
+        v3.calculateParams(inp);
+    }
+
+    function testNegativeMantissaHighSlotReverts() public {
+        OptimizerInput[8] memory inp;
+        inp[5] = OptimizerInput({mantissa: -1, shift: 0});
+        vm.expectRevert("negative mantissa");
+        v3.calculateParams(inp);
+    }
+
+    // ---- Basic output correctness ----
+
+    function testBearAtLowStaking() public view {
+        (uint256 ci, uint256 as_, uint24 aw, uint256 dd) =
+            v3.calculateParams(_inputs(50e16, 0));
+        assertEq(ci, 0);
+        assertEq(as_, 3e17);
+        assertEq(aw, 100);
+        assertEq(dd, 3e17);
+    }
+
+    function testBullAtHighStaking() public view {
+        (uint256 ci, uint256 as_, uint24 aw, uint256 dd) =
+            v3.calculateParams(_inputs(96e16, 0));
+        assertEq(ci, 0);
+        assertEq(as_, 1e18);
+        assertEq(aw, 20);
+        assertEq(dd, 1e18);
+    }
+}