feat: Add precision rounding for optimization values

Round design variables, objectives, and constraints to appropriate
decimal precision based on physical units (4 decimals for mm, degrees, MPa).

- Added _get_precision() method with unit-based precision mapping
- Round design variables when sampled from Optuna
- Round extracted results (objectives and constraints)
- Added units field to objectives in config files
- Tested: values now show 4 decimals instead of 17+

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

examples/bracket/optimization_config_stress_displacement.json

@@ -22,7 +22,8 @@
       "extractor": "stress_extractor",
       "metric": "max_von_mises",
       "direction": "minimize",
-      "weight": 10.0
+      "weight": 10.0,
+      "units": "MPa"
     }
   ],
   "constraints": [