refactor(editor): extract zone-level export handlers into cli_zone_export.cpp

Moves three per-zone visual / report exporters (--export-png,
--export-zone-deps-md, --export-zone-spawn-png) out of
main.cpp into a new cli_zone_export.{hpp,cpp} module.
- export-png renders heightmap + normal-map + zone-map PNG
  triplet for terrain previews
- export-zone-deps-md emits a GitHub-renderable model
  dependency table (PR-friendly counterpart to --list-zone-deps)
- export-zone-spawn-png plots top-down spawn distribution PNG
  bound to the zone's tile range

main.cpp shrinks by 267 lines (944 to 677) and crosses
below 700 lines on the way down. The previous --info-tilemap
dump used a 64x64 grid; this set complements it with content-
relative views.

tools/editor/cli_zone_export.hpp

@@ -0,0 +1,19 @@
+#pragma once
+
+namespace wowee {
+namespace editor {
+namespace cli {
+
+// Dispatch the zone-level visual / report export handlers —
+// render PNG previews of terrain, spawn distributions, and
+// Markdown dependency reports. All three operate per-zone.
+//   --export-png             heightmap + normals + zone-map PNG triplet
+//   --export-zone-deps-md    GitHub-renderable model-dependency table
+//   --export-zone-spawn-png  top-down spawn distribution PNG
+//
+// Returns true if matched; outRc holds the exit code.
+bool handleZoneExport(int& i, int argc, char** argv, int& outRc);
+
+} // namespace cli
+} // namespace editor
+} // namespace wowee