Add y_plane parameter to screen_to_world(), closes #245

screen_to_world() previously only intersected the Y=0 plane.
Now accepts an optional y_plane parameter (default 0.0) for
intersecting arbitrary horizontal planes.

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

tests/unit/test_screen_to_world_yplane.py

@@ -0,0 +1,54 @@
+"""Test screen_to_world y_plane parameter (issue #245)"""
+import mcrfpy
+import sys
+
+errors = []
+
+vp = mcrfpy.Viewport3D(pos=(0,0), size=(320,240))
+vp.set_grid_size(16, 16)
+# Position camera above the Y=0 plane
+vp.camera_pos = (0, 5, 5)
+vp.camera_target = (0, 0, 0)
+
+# Test 1: Default y_plane=0 works
+r1 = vp.screen_to_world(160, 120)
+if r1 is None:
+    errors.append("screen_to_world with default y_plane returned None")
+
+# Test 2: Explicit y_plane=0 matches default
+r2 = vp.screen_to_world(160, 120, y_plane=0.0)
+if r2 is None:
+    errors.append("screen_to_world with y_plane=0 returned None")
+elif r1 is not None:
+    for i in range(3):
+        if abs(r1[i] - r2[i]) > 0.001:
+            errors.append(f"Default and explicit y_plane=0 differ: {r1} vs {r2}")
+            break
+
+# Test 3: Different y_plane gives different result
+r3 = vp.screen_to_world(160, 120, y_plane=2.0)
+if r3 is None:
+    errors.append("screen_to_world with y_plane=2 returned None")
+elif r1 is not None:
+    if r1 == r3:
+        errors.append("y_plane=0 and y_plane=2 should give different results")
+
+# Test 4: y component matches y_plane
+if r3 is not None:
+    if abs(r3[1] - 2.0) > 0.001:
+        errors.append(f"y component should be 2.0 for y_plane=2.0, got {r3[1]}")
+
+# Test 5: Positional argument also works
+r4 = vp.screen_to_world(160, 120, 3.0)
+if r4 is None:
+    errors.append("Positional y_plane argument returned None")
+elif abs(r4[1] - 3.0) > 0.001:
+    errors.append(f"y component should be 3.0, got {r4[1]}")
+
+if errors:
+    for err in errors:
+        print(f"FAIL: {err}", file=sys.stderr)
+    sys.exit(1)
+else:
+    print("PASS: screen_to_world y_plane (issue #245)", file=sys.stderr)
+    sys.exit(0)