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/mod.rs

@@ -1153,12 +1153,6 @@ pub(super) struct OpenCADStudio {
     /// boundaries as well as after plugin-initiated writes.
     #[cfg(not(target_arch = "wasm32"))]
     last_plugin_document: Option<(u64, u64)>,
-    /// `(tab id, document generation)` last dispatched as `DocumentOpened` —
-    /// a tab's generation changes only when a whole document replaced its
-    /// content, so the opened notification fires once per load, never per
-    /// edit (API v9).
-    #[cfg(not(target_arch = "wasm32"))]
-    last_plugin_document_opened: Option<(u64, u64)>,
     /// External add-on packages found in the plugins folder, refreshed when the
     /// Plugin Manager opens.
     external_plugins: Vec<crate::plugin::external::ExternalPlugin>,
@@ -4838,8 +4832,6 @@ impl OpenCADStudio {
             last_plugin_selection: None,
             #[cfg(not(target_arch = "wasm32"))]
             last_plugin_document: None,
-            #[cfg(not(target_arch = "wasm32"))]
-            last_plugin_document_opened: None,
             external_plugins: Vec::new(),
             loaded_plugin_ids: rustc_hash::FxHashSet::default(),
             plugin_load_errors: rustc_hash::FxHashMap::default(),