refactor(editor): extract 12 composite mesh primitives into cli_gen_mesh.cpp

Moves the recently-added composite-prop mesh handlers (rock,
pillar, bridge, tower, house, fountain, statue, altar, portal,
archway, barrel, chest) into their own translation unit. These
12 handlers were the most contiguous block of similar-shaped
mesh code in main.cpp.

Other mesh handlers (--gen-mesh dispatcher, fence, tree, grid,
stairs, disc, tube, capsule, arch, pyramid, from-heightmap,
textured) still live in main.cpp and may be migrated in
subsequent batches.

main.cpp drops 24,270 → 22,681 lines (-1,589). Behavior
verified by re-running rock/chest/archway/fountain.

CMakeLists.txt

@@ -1306,6 +1306,7 @@ add_executable(wowee_editor
     tools/editor/cli_project_inventory.cpp
     tools/editor/cli_help.cpp
     tools/editor/cli_gen_texture.cpp
+    tools/editor/cli_gen_mesh.cpp
     tools/editor/editor_app.cpp
     tools/editor/editor_camera.cpp
     tools/editor/editor_viewport.cpp

tools/editor/cli_gen_mesh.hpp

@@ -0,0 +1,26 @@
+#pragma once
+
+namespace wowee {
+namespace editor {
+namespace cli {
+
+// Dispatch the procedural composite-prop mesh generators that
+// have been moved out of main.cpp:
+//   --gen-mesh-rock     --gen-mesh-pillar
+//   --gen-mesh-bridge   --gen-mesh-tower
+//   --gen-mesh-house    --gen-mesh-fountain
+//   --gen-mesh-statue   --gen-mesh-altar
+//   --gen-mesh-portal   --gen-mesh-archway
+//   --gen-mesh-barrel   --gen-mesh-chest
+//
+// Other mesh handlers (the `--gen-mesh <type>` dispatcher,
+// gen-mesh-fence/-tree/-grid/-stairs/-disc/-tube/-capsule/-arch/
+// -pyramid/-from-heightmap/-textured) still live in main.cpp
+// and may be migrated in subsequent batches.
+//
+// Returns true if matched; outRc holds the exit code.
+bool handleGenMesh(int& i, int argc, char** argv, int& outRc);
+
+} // namespace cli
+} // namespace editor
+} // namespace wowee