refactor(editor): extract mesh ⇄ heightmap I/O into cli_mesh_io.cpp

Moves the three mesh-PNG bridge handlers (--displace-mesh,
--gen-mesh-from-heightmap, --export-mesh-heightmap) out of
main.cpp into their own translation unit. These three are
distinct from the gen-mesh-* primitive generators in that
they read or write external image files rather than synthesize
geometry from parameters alone.

main.cpp drops 21,061 → 20,741 lines (-320). Behavior verified
by re-running gen-mesh-from-heightmap → validate-wom → and
displace-mesh on a fresh plane.

tools/editor/cli_mesh_io.hpp

@@ -0,0 +1,17 @@
+#pragma once
+
+namespace wowee {
+namespace editor {
+namespace cli {
+
+// Dispatch the mesh ⇄ heightmap I/O ops:
+//   --displace-mesh             (perturb verts along normal by PNG)
+//   --gen-mesh-from-heightmap   (generate WOM grid from PNG heightmap)
+//   --export-mesh-heightmap     (sample WOM Y to PNG heightmap)
+//
+// Returns true if matched; outRc holds the exit code.
+bool handleMeshIO(int& i, int argc, char** argv, int& outRc);
+
+} // namespace cli
+} // namespace editor
+} // namespace wowee