feat: Add Phase 2 & 3 physics extractors for multi-physics optimization

Phase 2 - Structural Analysis:
- extract_principal_stress: σ1, σ2, σ3 principal stresses from OP2
- extract_strain_energy: Element and total strain energy
- extract_spc_forces: Reaction forces at boundary conditions

Phase 3 - Multi-Physics:
- extract_temperature: Nodal temperatures from thermal OP2 (SOL 153/159)
- extract_temperature_gradient: Thermal gradient approximation
- extract_heat_flux: Element heat flux from thermal analysis
- extract_modal_mass: Modal effective mass from F06 (SOL 103)
- get_first_frequency: Convenience function for first natural frequency

Documentation:
- Updated SYS_12_EXTRACTOR_LIBRARY.md with E12-E18 specifications
- Updated NX_OPEN_AUTOMATION_ROADMAP.md marking Phase 3 complete
- Added test_phase3_extractors.py for validation

All extractors follow consistent API pattern returning Dict with
success, data, and error fields for robust error handling.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

optimization_engine/extractors/__init__.py

@@ -2,10 +2,25 @@
 
 Available extractors:
 - Displacement: extract_displacement
-- Stress: extract_solid_stress (von Mises)
+- Stress: extract_solid_stress (von Mises), extract_principal_stress
 - Frequency: extract_frequency
-- Mass: extract_mass_from_expression, extract_mass_from_op2
+- Mass (BDF): extract_mass_from_bdf
+- Mass (Expression): extract_mass_from_expression
+- Mass (Part): extract_part_mass_material, extract_part_mass, PartMassExtractor
+- Strain Energy: extract_strain_energy, extract_total_strain_energy
+- SPC Forces: extract_spc_forces, extract_total_reaction_force
 - Zernike: extract_zernike_from_op2, ZernikeExtractor (telescope mirrors)
+
+Phase 2 Extractors (2025-12-06):
+- Principal stress extraction (sigma1, sigma2, sigma3)
+- Strain energy extraction (element strain energy)
+- SPC forces extraction (reaction forces at boundary conditions)
+
+Phase 3 Extractors (2025-12-06):
+- Temperature extraction (thermal analysis: SOL 153/159)
+- Thermal gradient extraction
+- Heat flux extraction
+- Modal mass extraction (modal effective mass from F06)
 """
 
 # Zernike extractor for telescope mirror optimization
@@ -16,10 +31,90 @@ from optimization_engine.extractors.extract_zernike import (
     extract_zernike_relative_rms,
 )
 
+# Part mass and material extractor (from NX .prt files)
+from optimization_engine.extractors.extract_part_mass_material import (
+    extract_part_mass_material,
+    extract_part_mass,
+    extract_part_material,
+    PartMassExtractor,
+)
+
+# Von Mises stress extraction
+from optimization_engine.extractors.extract_von_mises_stress import (
+    extract_solid_stress,
+)
+
+# Principal stress extraction (Phase 2)
+from optimization_engine.extractors.extract_principal_stress import (
+    extract_principal_stress,
+    extract_max_principal_stress,
+    extract_min_principal_stress,
+)
+
+# Strain energy extraction (Phase 2)
+from optimization_engine.extractors.extract_strain_energy import (
+    extract_strain_energy,
+    extract_total_strain_energy,
+    extract_strain_energy_density,
+)
+
+# SPC forces / reaction forces extraction (Phase 2)
+from optimization_engine.extractors.extract_spc_forces import (
+    extract_spc_forces,
+    extract_total_reaction_force,
+    extract_reaction_component,
+    check_force_equilibrium,
+)
+
+# Temperature extraction (Phase 3)
+from optimization_engine.extractors.extract_temperature import (
+    extract_temperature,
+    extract_temperature_gradient,
+    extract_heat_flux,
+    get_max_temperature,
+)
+
+# Modal mass extraction (Phase 3)
+from optimization_engine.extractors.extract_modal_mass import (
+    extract_modal_mass,
+    extract_frequencies,
+    get_first_frequency,
+    get_modal_mass_ratio,
+)
+
 __all__ = [
+    # Part mass & material (from .prt)
+    'extract_part_mass_material',
+    'extract_part_mass',
+    'extract_part_material',
+    'PartMassExtractor',
+    # Stress extractors
+    'extract_solid_stress',
+    'extract_principal_stress',
+    'extract_max_principal_stress',
+    'extract_min_principal_stress',
+    # Strain energy
+    'extract_strain_energy',
+    'extract_total_strain_energy',
+    'extract_strain_energy_density',
+    # SPC forces / reactions
+    'extract_spc_forces',
+    'extract_total_reaction_force',
+    'extract_reaction_component',
+    'check_force_equilibrium',
     # Zernike (telescope mirrors)
     'ZernikeExtractor',
     'extract_zernike_from_op2',
     'extract_zernike_filtered_rms',
     'extract_zernike_relative_rms',
+    # Temperature (Phase 3 - thermal)
+    'extract_temperature',
+    'extract_temperature_gradient',
+    'extract_heat_flux',
+    'get_max_temperature',
+    # Modal mass (Phase 3 - dynamics)
+    'extract_modal_mass',
+    'extract_frequencies',
+    'get_first_frequency',
+    'get_modal_mass_ratio',
 ]