HeightMap: add threshold operations that return new HeightMaps (closes #197)

Add three methods that create NEW HeightMap objects:
- threshold(range): preserve original values where in range, 0.0 elsewhere
- threshold_binary(range, value=1.0): set uniform value where in range
- inverse(): return (1.0 - value) for each cell

These operations are immutable - they preserve the original HeightMap.
Useful for masking operations with Grid.apply_threshold/apply_ranges.

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

src/PyHeightMap.h

@@ -40,6 +40,11 @@ public:
     static PyObject* min_max(PyHeightMapObject* self, PyObject* Py_UNUSED(args));
     static PyObject* count_in_range(PyHeightMapObject* self, PyObject* args);
 
+    // Threshold operations (#197) - return NEW HeightMaps
+    static PyObject* threshold(PyHeightMapObject* self, PyObject* args);
+    static PyObject* threshold_binary(PyHeightMapObject* self, PyObject* args, PyObject* kwds);
+    static PyObject* inverse(PyHeightMapObject* self, PyObject* Py_UNUSED(args));
+
     // Subscript support for hmap[x, y] syntax
     static PyObject* subscript(PyHeightMapObject* self, PyObject* key);