fix: mass extraction NaN in Hydrotech Beam DOE — two bugs

Bug 1 — Journal (solve_simulation.py simple workflow):
  Expression lookup for p173 fails silently for derived/measurement
  expressions, so _temp_mass.txt was never written. Added MeasureManager
  fallback via extract_part_mass() (already used in assembly workflow).

Bug 2 — Extractor (extract_mass_from_expression.py):
  Journal writes 'p173=<value>' format but extractor tried float() on
  the whole content including 'p173='. Added key=value parsing.

Defense in depth — nx_interface.py:
  Added stdout parsing fallback: if _temp_mass.txt still missing, parse
  mass from journal output captured via solver.py stdout passthrough.

Files changed:
  - optimization_engine/nx/solve_simulation.py — MeasureManager fallback
  - optimization_engine/extractors/extract_mass_from_expression.py — key=value parse
  - optimization_engine/nx/solver.py — include stdout in result dict
  - projects/hydrotech-beam/studies/01_doe_landscape/nx_interface.py — stdout fallback

Tags: hydrotech-beam, mass-extraction

optimization_engine/extractors/extract_mass_from_expression.py

@@ -42,13 +42,19 @@ def extract_mass_from_expression(prt_file: Path, expression_name: str = "p173")
     # Read mass from file
     try:
         with open(mass_file, 'r') as f:
-            mass_kg = float(f.read().strip())
+            content = f.read().strip()
+
+        # Handle key=value format (e.g., "p173=1185.767")
+        if '=' in content:
+            content = content.split('=', 1)[1]
+
+        mass_kg = float(content)
 
         print(f"[OK] Mass from {expression_name}: {mass_kg:.6f} kg ({mass_kg * 1000:.2f} g)")
         return mass_kg
 
     except ValueError as e:
-        raise ValueError(f"Could not parse mass from {mass_file}: {e}")
+        raise ValueError(f"Could not parse mass from {mass_file} (content: {content!r}): {e}")
     except Exception as e:
         raise RuntimeError(f"Failed to read mass file: {e}")