MCP: report render progress and handle cancellation

Keep serving during long renders, recover failed workers, and remove only files opened by the interrupted export. Cover progress, cancellation, EOF and output ownership with regression tests.

Co-Authored-By: OpenAI Codex <noreply@openai.com>
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/engine/src/render_queue.rs

@@ -58,7 +58,8 @@ pub trait Exporter: Send + Sync {
     /// Formats this build can write.
     fn formats(&self) -> Vec<OutputFormat>;
     /// Render and write one item (blocking). `progress(done, total)` returns `false` to cancel;
-    /// a cancelled export returns `Err("cancelled")`.
+    /// a cancelled export removes the files it opened for that output and returns `Err("cancelled")`.
+    /// Completed earlier queue items and unrelated files must be preserved.
     fn export(&self, job: &ExportJob, progress: &mut dyn FnMut(u64, u64) -> bool) -> Result<ExportResult, String>;
     /// [`Exporter::export`] as a future: renders whose GPU readbacks are deferred (a browser
     /// job worker's WebGPU device) await the device between a frame's passes. The default runs
@@ -494,7 +495,22 @@ impl Session {
             let sh = shared.clone();
             let thread = std::thread::Builder::new()
                 .name("render-queue".into())
-                .spawn(move || effectcraft_render::passes::block_on(run_work(work, &sh, &mut |_| {})))
+                .spawn(move || {
+                    let pending: Vec<u64> = work.items.iter().filter_map(|modules| modules.first().map(|(item, _)| item.id)).collect();
+                    if let Err(error) = crate::guard::guarded("render worker", || {
+                        effectcraft_render::passes::block_on(run_work(work, &sh, &mut |_| {}));
+                        Ok(())
+                    }) {
+                        let completed = lock(&sh.state).items_done;
+                        let mut updates = lock(&sh.updates);
+                        for id in pending.into_iter().skip(completed) {
+                            if !updates.iter().any(|u| matches!(u, ItemUpdate::Finished { id: done, .. } if *done == id)) {
+                                updates.push(ItemUpdate::Finished { id, status: RenderStatus::Failed(error.to_string()), seconds: 0.0, output: None });
+                            }
+                        }
+                        lock(&sh.state).finished = true;
+                    }
+                })
                 .map_err(|e| e.to_string())?;
             self.render_job = Some(RenderJob { shared, thread: Some(thread), remote: None });
         }