Per-command timings for script runs and batches (#1868)

Thanks @DuBearte.

crates/mcp/src/backend.rs

@@ -21,6 +21,11 @@ pub trait Backend {
     fn design_path(&self) -> Option<String> {
         None
     }
+    /// Commands run in this process, on this thread (the kernel's time in them can be measured
+    /// here, #1833).
+    fn in_process(&self) -> bool {
+        false
+    }
 }
 
 /// Longest reply line accepted from the app.
@@ -308,4 +313,7 @@ impl Backend for Headless {
     fn design_path(&self) -> Option<String> {
         self.session.path.clone()
     }
+    fn in_process(&self) -> bool {
+        true
+    }
 }