feat(commands): add REGION (closed boundaries -> region entities)

Convert selected closed polylines and circles into Region entities,
one wire loop each (reuses acadrust Region/Wire). COMMANDS.md: 215/262.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/commands/mod.rs

@@ -342,6 +342,8 @@ inventory::submit!(crate::command::CommandRegistration {
         "QC",
         "HYPERLINK",
         "XOPEN",
+        "REGION",
+        "REG",
         // Slice/section + interference + press-pull/thicken + 3D transforms + wall/pyramid.
         "SLICE",
         "SL",