feat(canvas): Custom extractor components, migrator, and MCP spec tools

Canvas Components:
- CustomExtractorNode.tsx: Node for custom Python extractors
- CustomExtractorPanel.tsx: Configuration panel for custom extractors
- ConnectionStatusIndicator.tsx: WebSocket status display
- atomizer-spec.ts: TypeScript types for AtomizerSpec v2.0

Config:
- migrator.py: Legacy config to AtomizerSpec v2.0 migration
- Updated __init__.py exports for config and extractors

MCP Tools:
- spec.ts: MCP tools for spec manipulation
- index.ts: Tool registration updates

optimization_engine/extractors/__init__.py

@@ -11,6 +11,7 @@ Available extractors:
 - SPC Forces: extract_spc_forces, extract_total_reaction_force
 - Zernike: extract_zernike_from_op2, ZernikeExtractor (telescope mirrors)
 - Part Introspection: introspect_part (comprehensive NX .prt analysis)
+- Custom: CustomExtractorLoader for user-defined Python extractors
 
 Phase 2 Extractors (2025-12-06):
 - Principal stress extraction (sigma1, sigma2, sigma3)
@@ -25,6 +26,10 @@ Phase 3 Extractors (2025-12-06):
 
 Phase 4 Extractors (2025-12-19):
 - Part Introspection (E12): Comprehensive .prt analysis (expressions, mass, materials, attributes, groups, features)
+
+Phase 5 Extractors (2026-01-17):
+- Custom Extractor Loader: Dynamic loading and execution of user-defined Python extractors
+  from AtomizerSpec v2.0 (sandboxed execution with security validation)
 """
 
 # Zernike extractor for telescope mirror optimization (standard Z-only method)
@@ -119,6 +124,26 @@ from optimization_engine.extractors.introspect_part import (
     print_introspection_summary,
 )
 
+# Custom extractor loader (Phase 5) - dynamic Python extractors from AtomizerSpec v2.0
+from optimization_engine.extractors.custom_extractor_loader import (
+    CustomExtractor,
+    CustomExtractorLoader,
+    CustomExtractorContext,
+    ExtractorSecurityError,
+    ExtractorValidationError,
+    load_custom_extractors,
+    execute_custom_extractor,
+    validate_custom_extractor,
+)
+
+# Spec extractor builder - builds extractors from AtomizerSpec
+from optimization_engine.extractors.spec_extractor_builder import (
+    SpecExtractorBuilder,
+    build_extractors_from_spec,
+    get_extractor_outputs,
+    list_available_builtin_extractors,
+)
+
 __all__ = [
     # Part mass & material (from .prt)
     'extract_part_mass_material',
@@ -174,4 +199,18 @@ __all__ = [
     'get_expressions_dict',
     'get_expression_value',
     'print_introspection_summary',
+    # Custom extractor loader (Phase 5)
+    'CustomExtractor',
+    'CustomExtractorLoader',
+    'CustomExtractorContext',
+    'ExtractorSecurityError',
+    'ExtractorValidationError',
+    'load_custom_extractors',
+    'execute_custom_extractor',
+    'validate_custom_extractor',
+    # Spec extractor builder
+    'SpecExtractorBuilder',
+    'build_extractors_from_spec',
+    'get_extractor_outputs',
+    'list_available_builtin_extractors',
 ]