fix(P2-10): is_placeholder now reflects actual LLM driver availability

Previously hardcoded to false in Tauri bridge. Now checks whether
kernel provided a driver before building classroom, correctly flagging
placeholder content when LLM is unavailable.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

desktop/src-tauri/src/classroom_commands/generate.rs

@@ -129,12 +129,12 @@ pub async fn classroom_generate(
     }
 
     // Get LLM driver from kernel if available, otherwise use placeholder mode
-    let pipeline = {
+    let (pipeline, has_driver) = {
         let ks = kernel_state.lock().await;
         if let Some(kernel) = ks.as_ref() {
-            GenerationPipeline::with_driver(kernel.driver(), kernel.config().model().to_string())
+            (GenerationPipeline::with_driver(kernel.driver(), kernel.config().model().to_string()), true)
         } else {
-            GenerationPipeline::new()
+            (GenerationPipeline::new(), false)
         }
     };
 
@@ -211,7 +211,7 @@ pub async fn classroom_generate(
             source_document: kernel_request.document.map(|_| "user_document".to_string()),
             model: None,
             version: "2.0.0".to_string(),
-            is_placeholder: false, // P2-10: Tauri layer always has a driver
+            is_placeholder: !has_driver, // P2-10: true when no LLM driver available
             custom: serde_json::Map::new(),
         },
     };

desktop/src/types/classroom.ts

@@ -109,6 +109,8 @@ export interface ClassroomMetadata {
   sourceDocument?: string;
   model?: string;
   version: string;
+  /** P2-10: marks content generated by placeholder fallback when LLM fails */
+  is_placeholder?: boolean;
   custom: Record<string, unknown>;
 }