Remove Unused Project Includes and Forward-Declare Where a Type Is Only Referenced

Generated with include-what-you-use and applied conservatively. Only OrcaSlicer's own headers, the ones under src/ and tests/, are removed or forward-declared; standard-library and third-party includes are left alone. An include is removed only when both the Release and the Debug configuration leave it unused, never from inside a conditional block, and never from a file with platform-specific blocks, which only gain includes. Files whose only use of a header sits behind a feature or debug macro (libvgcode's OpenGL ES and marker code, the ARACHNE/TESTS_EXPORT_SVGS debug output) keep their includes.

clonable_ptr.hpp gains #pragma once; it had no include guard and was only safe while Config.hpp was its sole includer.

tests/fff_print/test_slicing_pipeline_hook.cpp

@@ -496,6 +496,9 @@ TEST_CASE("refreshing lslices after a slice mutation makes islands track the geo
 }
 
 #include <random>   // deterministic RNG for the fuzzy-skin analogue below
+#include "libslic3r/ExPolygon.hpp"
+#include "libslic3r/Polygon.hpp"
+#include "libslic3r/SurfaceCollection.hpp"
 
 // Fuzzy skin applied to the slice contours at the Slice boundary, matching what the Fuzzy
 // Slices sample (sandboxes/orca_fuzzy_slices_plugin_any.py) does: resample every ring at