plugin: announce DocumentOpened once per load, not per tab switch

The guard remembered only the last announced (tab, generation), so
switching A -> B -> A announced A's load again. Each tab now carries
its own pending flag, set when a document is installed and cleared by
the boundary dispatch.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/control/sheets.rs

@@ -466,8 +466,8 @@ impl OpenCADStudio {
         self.tabs[i].current_path = None;
         self.tabs[i].is_start = false;
         self.tabs[i].dirty = true;
-        // A whole document just became resident in the tab (API v9): mint a
-        // fresh generation so the boundary announces `DocumentOpened` once.
+        // A whole document just became resident in the tab (API v9): the
+        // boundary announces `DocumentOpened` once.
         self.tabs[i].note_document_loaded();
         Ok(purged)
     }