Fix Entity3D.viewport returning None, closes #244

The root cause was PyViewport3DType being declared `static` in
Viewport3D.h, creating per-translation-unit copies. Entity3D.cpp's
copy was never passed through PyType_Ready, causing segfaults when
tp_alloc was called.

Changed `static` to `inline` (matching PyEntity3DType and
PyModel3DType patterns), and implemented get_viewport using the
standard type->tp_alloc pattern.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/unit/test_entity3d_viewport.py

@@ -0,0 +1,44 @@
+"""Test Entity3D.viewport property (issue #244)"""
+import mcrfpy
+import sys
+
+errors = []
+
+# Test 1: Detached entity returns None
+e = mcrfpy.Entity3D(pos=(0,0), scale=1.0)
+if e.viewport is not None:
+    errors.append("Detached entity viewport should be None")
+
+# Test 2: Attached entity returns Viewport3D
+vp = mcrfpy.Viewport3D(pos=(0,0), size=(100,100))
+vp.set_grid_size(16, 16)
+e2 = mcrfpy.Entity3D(pos=(5,5), scale=1.0)
+vp.entities.append(e2)
+v = e2.viewport
+if v is None:
+    errors.append("Attached entity viewport should not be None")
+elif type(v).__name__ != 'Viewport3D':
+    errors.append(f"Expected Viewport3D, got {type(v).__name__}")
+
+# Test 3: Viewport properties are accessible
+if v is not None:
+    try:
+        _ = v.w
+        _ = v.h
+    except Exception as ex:
+        errors.append(f"Viewport properties not accessible: {ex}")
+
+# Test 4: Multiple entities share same viewport
+e3 = mcrfpy.Entity3D(pos=(3,3), scale=1.0)
+vp.entities.append(e3)
+v2 = e3.viewport
+if v2 is None:
+    errors.append("Second entity viewport should not be None")
+
+if errors:
+    for err in errors:
+        print(f"FAIL: {err}", file=sys.stderr)
+    sys.exit(1)
+else:
+    print("PASS: Entity3D.viewport (issue #244)", file=sys.stderr)
+    sys.exit(0)