feat(canvas): Add file browser, introspection, and improve node flow

Phase 1-7 of Canvas V4 Ralph Loop implementation:

Backend:
- Add /api/files routes for browsing model files
- Add /api/nx routes for NX model introspection
- Add NXIntrospector service to discover expressions and extractors
- Add health check with database status

Frontend:
- Add FileBrowser component for selecting .sim/.prt/.fem files
- Add IntrospectionPanel to discover expressions and extractors
- Update NodeConfigPanel with browse and introspect buttons
- Update schema with NODE_HANDLES for proper flow direction
- Update validation for correct DesignVar -> Model -> Solver flow
- Update useCanvasStore.addNode() to accept custom data

Flow correction: Design Variables now connect TO Model (as source),
not FROM Model. This matches the actual data flow in optimization.

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

atomizer-dashboard/frontend/src/lib/canvas/schema.ts

@@ -89,14 +89,63 @@ export interface CanvasEdge {
   targetHandle?: string;
 }
 
-// Valid connections
+// Valid connections - defines what a node can connect TO (as source)
+// Flow: DesignVar -> Model -> Solver -> Extractor -> Objective/Constraint -> Algorithm -> Surrogate
 export const VALID_CONNECTIONS: Record<NodeType, NodeType[]> = {
-  model: ['solver', 'designVar'],
-  solver: ['extractor'],
-  designVar: ['model'],
-  extractor: ['objective', 'constraint'],
-  objective: ['algorithm'],
-  constraint: ['algorithm'],
-  algorithm: ['surrogate'],
-  surrogate: [],
+  model: ['solver'],           // Model outputs to Solver
+  solver: ['extractor'],       // Solver outputs to Extractor
+  designVar: ['model'],        // DesignVar outputs to Model (expressions feed into model)
+  extractor: ['objective', 'constraint'], // Extractor outputs to Objective/Constraint
+  objective: ['algorithm'],    // Objective outputs to Algorithm
+  constraint: ['algorithm'],   // Constraint outputs to Algorithm
+  algorithm: ['surrogate'],    // Algorithm outputs to Surrogate
+  surrogate: [],               // Surrogate is terminal
+};
+
+// Node handle configuration for proper flow direction
+export interface HandleConfig {
+  id: string;
+  label?: string;
+}
+
+export interface NodeHandleConfig {
+  inputs: HandleConfig[];
+  outputs: HandleConfig[];
+}
+
+// Define handles for each node type
+// Flow: DesignVar(s) -> Model -> Solver -> Extractor(s) -> Objective(s) -> Algorithm
+export const NODE_HANDLES: Record<NodeType, NodeHandleConfig> = {
+  model: {
+    inputs: [{ id: 'params', label: 'Parameters' }],  // Receives from DesignVars
+    outputs: [{ id: 'sim', label: 'Simulation' }],    // Sends to Solver
+  },
+  solver: {
+    inputs: [{ id: 'model', label: 'Model' }],        // Receives from Model
+    outputs: [{ id: 'results', label: 'Results' }],   // Sends to Extractors
+  },
+  designVar: {
+    inputs: [],                                        // No inputs - this is a source
+    outputs: [{ id: 'value', label: 'Value' }],       // Sends to Model
+  },
+  extractor: {
+    inputs: [{ id: 'results', label: 'Results' }],    // Receives from Solver
+    outputs: [{ id: 'value', label: 'Value' }],       // Sends to Objective/Constraint
+  },
+  objective: {
+    inputs: [{ id: 'value', label: 'Value' }],        // Receives from Extractor
+    outputs: [{ id: 'objective', label: 'Objective' }], // Sends to Algorithm
+  },
+  constraint: {
+    inputs: [{ id: 'value', label: 'Value' }],        // Receives from Extractor
+    outputs: [{ id: 'constraint', label: 'Constraint' }], // Sends to Algorithm
+  },
+  algorithm: {
+    inputs: [{ id: 'objectives', label: 'Objectives' }], // Receives from Objectives/Constraints
+    outputs: [{ id: 'algo', label: 'Algorithm' }],    // Sends to Surrogate
+  },
+  surrogate: {
+    inputs: [{ id: 'algo', label: 'Algorithm' }],     // Receives from Algorithm
+    outputs: [],                                       // No outputs - this is a sink
+  },
 };