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/update/file.rs

@@ -2276,9 +2276,9 @@ impl OpenCADStudio {
                 .set(crate::app::OPEN_PHASE_FINALIZING, 10000, 1, 1);
         }
         self.opening.take();
-        // The document and its derived caches have settled: mark the tab's
-        // generation so the next message boundary announces `DocumentOpened`
-        // to native plugins exactly once for this load (API v9).
+        // The document and its derived caches have settled: the next message
+        // boundary announces `DocumentOpened` to native plugins exactly once
+        // for this load (API v9).
         self.tabs[i].note_document_loaded();
         // The tab list just changed (and the active tab with it): remember
         // it for the next boot's restore flow (web session memory).