Part 3.1: refactor BeltTransform pipeline

add BeltGCodeWriter

add BeltGCode

consolidate changes into shared classes for BeltGcode

src/libslic3r/BeltGCode.hpp

@@ -0,0 +1,28 @@
+#pragma once
+
+#include "GCode.hpp"
+
+namespace Slic3r {
+
+// Belt-printer-specific GCode export.
+//
+// Inherits from GCode and overrides virtual hooks to:
+// - Create a BeltGCodeWriter instead of a plain GCodeWriter
+// - Write belt configuration to the G-code header
+// - Update per-axis origin snap when switching instances
+// - Disable arc fitting (G2/G3 not supported on belt printers)
+class BeltGCode : public GCode
+{
+protected:
+    void init_belt_writer(Print &print, bool is_bbl_printers) override;
+    void write_belt_header(GCodeOutputStream &file, const Print &print) override;
+    void on_set_origin(const PrintObject *obj, const Point &inst_shift) override;
+    bool should_disable_arc_fitting() const override { return true; }
+
+private:
+    // Per-axis origin snap config (set during init, used in on_set_origin).
+    bool   m_origin_snap[3]        = {false, false, false};
+    double m_origin_snap_offset[3] = {0., 0., 0.};
+};
+
+} // namespace Slic3r