Fix KiCAD 9.0 API compatibility for board, export, and view commands

This commit resolves critical KiCAD 9.0 API compatibility issues identified
through comprehensive testing and code audit.

## Major Fixes

### board/size.py - Board Outline Creation
- **Issue:** Stub implementation didn't create actual board edges
- **Fix:** Delegate to BoardOutlineCommands for proper Edge.Cuts geometry creation
- **Impact:** set_board_size() now creates actual rectangular board outlines
- **Tested:** 100x80mm and 4x3 inch board creation validated

### board/view.py - Board Extents Query
- **Issue:** get_board_extents() command not implemented
- **Fix:** New implementation returns complete bounding box data
- **Returns:** left, top, right, bottom, width, height, center coordinates
- **Tested:** Successfully returns 93.55mm × 212.05mm for test board

### board/__init__.py - Delegation Wiring
- **Fix:** Added get_board_extents() delegation to BoardViewCommands
- **Impact:** Completes board query API surface

### export.py - 3D Model Export
- **Issue:** Get3DViewer() doesn't work in headless KiCAD 9.0
- **Fix:** Replace with kicad-cli subprocess calls for STEP/VRML export
- **New:** _find_kicad_cli() helper for cross-platform CLI detection
- **Features:**
  - STEP export with component/copper/silkscreen/soldermask options
  - VRML export with configurable units
  - 5-minute timeout for large boards
  - Proper error handling and validation
- **Tested:**
  - STEP export: 144.65 MB (full), 114.61 MB (board-only)
  - VRML export: 60.69 MB
  - All exports successful on 415-component board

## Test Results

All changes validated with test_kicad_9_fixes.py:
- ✅ Board outline creation (mm and inch units)
- ✅ Board extents query
- ✅ STEP 3D export (full and board-only)
- ✅ VRML 3D export
- ✅ Error handling and validation

## Compatibility

- **KiCAD Version:** 9.0.6 (tested)
- **Backward Compatible:** Yes (kicad-cli available in KiCAD 8.0+)
- **Platform:** Windows, macOS, Linux

## Breaking Changes

None - all changes are additions or fixes to broken functionality.

## Related

- Complements previous fixes: design_rules.py, component.py, layers.py
- Part of comprehensive KiCAD 9.0 compatibility effort
- Documentation: PYTHON_AUDIT_REPORT_FINAL.md, TEST_RESULTS.md

Fixes #<issue-number-if-applicable>

python/commands/board/__init__.py

@@ -75,3 +75,8 @@ class BoardCommands:
         """Get a 2D image of the PCB"""
         self.view_commands.board = self.board
         return self.view_commands.get_board_2d_view(params)
+
+    def get_board_extents(self, params: Dict[str, Any]) -> Dict[str, Any]:
+        """Get the bounding box extents of the board"""
+        self.view_commands.board = self.board
+        return self.view_commands.get_board_extents(params)