feat: Replace SDF kernel with OpenCASCADE, VTK with pygfx

Major architecture migration:

- Remove SDF-based geometry kernel (sdf/)
- Remove VTK renderer (drawing_modules/)
- Remove old mesh modules (mesh_modules/)

New components:
- geometry/base.py: Abstract geometry kernel interface
- geometry_occ/kernel.py: OpenCASCADE implementation via CadQuery/OCP
- geometry_occ/sketch.py: 2D sketching with constraint solving
- rendering/base.py: Abstract renderer interface
- rendering/pygfx_renderer.py: WebGPU-based renderer
- models/data_model.py: Project, Component, Sketch, Body classes
- main.py: New Qt-based application

Features:
- STEP/IGES import/export
- Exact BRep geometry (vs approximate SDF mesh)
- Parametric sketching with constraints
- Boolean operations (union, difference, intersection)
- Fillet and chamfer operations
- Modern pygfx renderer (~30MB vs VTK ~200MB)

Dependencies:
- cadquery >= 2.4
- ocp >= 7.9.3
- pygfx >= 0.7.0
- wgpu >= 0.19.0
- PySide6 >= 6.9.0

src/fluency/geometry/__init__.py

@@ -0,0 +1,19 @@
+"""Geometry abstraction layer for Fluency CAD."""
+
+from fluency.geometry.base import (
+    Point2D,
+    Point3D,
+    GeometryObject,
+    GeometryKernel,
+    SketchInterface,
+    SketchEntity,
+)
+
+__all__ = [
+    "Point2D",
+    "Point3D",
+    "GeometryObject",
+    "GeometryKernel",
+    "SketchInterface",
+    "SketchEntity",
+]