feat: Add sub-part introspection and existing FEA results UI

Backend:
- GET /nx/parts - List all .prt files in model directory
- GET /nx/introspect/{part_name} - Introspect a specific part file
  (e.g., M1_Blank.prt instead of just the assembly)
- Each part gets its own cache file (_introspection_{stem}.json)

Frontend IntrospectionPanel:
- Add 'FEA Results' section showing existing OP2/F06 sources
- Green checkmark when results exist, shows recommended source
- Expand file_deps and fea_results sections by default
- Add CheckCircle2 and Database icons

This allows introspecting component parts that contain the actual
design variable expressions (e.g., M1_Blank has 56 expressions
while the assembly ASSY_M1 only has 5).

atomizer-dashboard/backend/api/routes/optimization.py

@@ -5280,3 +5280,155 @@ async def get_nx_expressions(study_id: str):
         raise
     except Exception as e:
         raise HTTPException(status_code=500, detail=f"Failed to get expressions: {str(e)}")
+
+
+@router.get("/studies/{study_id}/nx/introspect/{part_name}")
+async def introspect_specific_part(study_id: str, part_name: str, force: bool = False):
+    """
+    Introspect a specific .prt file in the model directory.
+
+    Use this to get expressions from component parts (e.g., M1_Blank.prt)
+    rather than just the main assembly.
+
+    Args:
+        study_id: Study identifier
+        part_name: Name of the part file (e.g., "M1_Blank.prt" or "M1_Blank")
+        force: Force re-introspection even if cached
+
+    Returns:
+        JSON with introspection results for the specific part
+    """
+    try:
+        study_dir = resolve_study_path(study_id)
+        if not study_dir.exists():
+            raise HTTPException(status_code=404, detail=f"Study {study_id} not found")
+
+        # Find model directory
+        model_dir = None
+        for possible_dir in [
+            study_dir / "1_setup" / "model",
+            study_dir / "1_model",
+            study_dir / "model",
+            study_dir / "1_setup",
+        ]:
+            if possible_dir.exists():
+                model_dir = possible_dir
+                break
+
+        if model_dir is None:
+            raise HTTPException(status_code=404, detail=f"No model directory found for {study_id}")
+
+        # Normalize part name
+        if not part_name.lower().endswith(".prt"):
+            part_name = part_name + ".prt"
+
+        # Find the part file (case-insensitive)
+        prt_file = None
+        for f in model_dir.glob("*.prt"):
+            if f.name.lower() == part_name.lower():
+                prt_file = f
+                break
+
+        if prt_file is None:
+            # List available parts for helpful error
+            available = [f.name for f in model_dir.glob("*.prt")]
+            raise HTTPException(
+                status_code=404, detail=f"Part '{part_name}' not found. Available: {available}"
+            )
+
+        # Check cache
+        cache_file = model_dir / f"_introspection_{prt_file.stem}.json"
+        if cache_file.exists() and not force:
+            try:
+                with open(cache_file, "r") as f:
+                    cached = json.load(f)
+                return {
+                    "study_id": study_id,
+                    "part_name": prt_file.name,
+                    "cached": True,
+                    "introspection": cached,
+                }
+            except:
+                pass
+
+        # Run introspection
+        try:
+            from optimization_engine.extractors.introspect_part import introspect_part
+
+            result = introspect_part(str(prt_file), str(model_dir), verbose=False)
+
+            # Cache results
+            with open(cache_file, "w") as f:
+                json.dump(result, f, indent=2)
+
+            return {
+                "study_id": study_id,
+                "part_name": prt_file.name,
+                "cached": False,
+                "introspection": result,
+            }
+
+        except ImportError:
+            raise HTTPException(status_code=500, detail="introspect_part module not available")
+        except Exception as e:
+            raise HTTPException(status_code=500, detail=f"Introspection failed: {str(e)}")
+
+    except HTTPException:
+        raise
+    except Exception as e:
+        raise HTTPException(status_code=500, detail=f"Failed to introspect part: {str(e)}")
+
+
+@router.get("/studies/{study_id}/nx/parts")
+async def list_model_parts(study_id: str):
+    """
+    List all .prt files in the study's model directory.
+
+    Returns:
+        JSON with list of available parts that can be introspected
+    """
+    try:
+        study_dir = resolve_study_path(study_id)
+        if not study_dir.exists():
+            raise HTTPException(status_code=404, detail=f"Study {study_id} not found")
+
+        # Find model directory
+        model_dir = None
+        for possible_dir in [
+            study_dir / "1_setup" / "model",
+            study_dir / "1_model",
+            study_dir / "model",
+            study_dir / "1_setup",
+        ]:
+            if possible_dir.exists():
+                model_dir = possible_dir
+                break
+
+        if model_dir is None:
+            raise HTTPException(status_code=404, detail=f"No model directory found for {study_id}")
+
+        # Collect all part files
+        parts = []
+        for f in sorted(model_dir.glob("*.prt")):
+            is_idealized = "_i.prt" in f.name.lower()
+            parts.append(
+                {
+                    "name": f.name,
+                    "stem": f.stem,
+                    "is_idealized": is_idealized,
+                    "size_kb": round(f.stat().st_size / 1024, 1),
+                    "has_cache": (model_dir / f"_introspection_{f.stem}.json").exists(),
+                }
+            )
+
+        return {
+            "study_id": study_id,
+            "model_dir": str(model_dir),
+            "parts": parts,
+            "count": len(parts),
+        }
+
+    except HTTPException:
+        raise
+    except Exception as e:
+        raise HTTPException(status_code=500, detail=f"Failed to list parts: {str(e)}")