HeightMap: add terrain generation methods (closes #195)

Add seven terrain generation methods wrapping libtcod heightmap functions:
- add_hill(center, radius, height): Add smooth hill
- dig_hill(center, radius, depth): Dig crater (use negative depth)
- add_voronoi(num_points, coefficients, seed): Voronoi-based features
- mid_point_displacement(roughness, seed): Diamond-square terrain
- rain_erosion(drops, erosion, sedimentation, seed): Erosion simulation
- dig_bezier(points, start_radius, end_radius, start_depth, end_depth): Carve paths
- smooth(iterations): Average neighboring cells

All methods return self for chaining. Includes 24 unit tests.

Note: dig_hill and dig_bezier use libtcod's "dig" semantics - use negative
depth values to actually dig below current terrain level.

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

src/PyHeightMap.h

@@ -45,6 +45,15 @@ public:
     static PyObject* threshold_binary(PyHeightMapObject* self, PyObject* args, PyObject* kwds);
     static PyObject* inverse(PyHeightMapObject* self, PyObject* Py_UNUSED(args));
 
+    // Terrain generation methods (#195) - mutate self, return self for chaining
+    static PyObject* add_hill(PyHeightMapObject* self, PyObject* args, PyObject* kwds);
+    static PyObject* dig_hill(PyHeightMapObject* self, PyObject* args, PyObject* kwds);
+    static PyObject* add_voronoi(PyHeightMapObject* self, PyObject* args, PyObject* kwds);
+    static PyObject* mid_point_displacement(PyHeightMapObject* self, PyObject* args, PyObject* kwds);
+    static PyObject* rain_erosion(PyHeightMapObject* self, PyObject* args, PyObject* kwds);
+    static PyObject* dig_bezier(PyHeightMapObject* self, PyObject* args, PyObject* kwds);
+    static PyObject* smooth(PyHeightMapObject* self, PyObject* args, PyObject* kwds);
+
     // Subscript support for hmap[x, y] syntax
     static PyObject* subscript(PyHeightMapObject* self, PyObject* key);