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/src/IOptimizer.sol

@@ -3,10 +3,9 @@ pragma solidity ^0.8.19;
 
 /**
  * @notice Dyadic rational input: mantissa × 2^(-shift).
- *         shift is reserved for future use and MUST be 0.  All current
- *         Optimizer implementations (Optimizer, OptimizerV3, OptimizerV3Push3)
- *         require shift == 0 and revert otherwise.  When shift == 0 (as
- *         produced by _toDyadic), value == mantissa.
+ *         shift is reserved for future use and MUST be 0.  All production
+ *         Optimizer implementations require shift == 0 and revert otherwise.
+ *         When shift == 0 (as produced by _toDyadic), value == mantissa.
  */
 struct OptimizerInput {
     int256 mantissa;