fix: crash on startup from stale self-pointers after app move

The container/splitbox widgets capture raw self-pointers during
rebuild_layout. rebuild_layout was first called inside new(), before
the FilesystemApp value is moved into the engine, so those pointers
dangled once the value relocated — a SIGSEGV in BrowseContainer::set_parent
that previously only avoided crashing by return-value-optimization luck
(disturbed by the prior struct field removal).

Refresh the container child pointers at the top of every rebuild, and
defer the first rebuild to the engine's first frame (post-move) so the
splitbox captures the container at its final address.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

 src/main.rs | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/src/main.rs b/src/main.rs
index 1efb294..20d9211 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -369,6 +369,16 @@ impl FilesystemApp {
     }
 
     fn rebuild_layout(&mut self) {
+        // Refresh the container child pointers on every rebuild. They point into
+        // self.browse / self.network, whose addresses change when the app value is
+        // first moved out of new(); re-taking them here (where self is at its final
+        // address) keeps them valid regardless of return-value optimization.
+        self.browse_container.breadcrumb = &mut self.browse.breadcrumb;
+        self.browse_container.list_box = &mut self.browse.list_box;
+        self.browse_container.save_name_box = &mut self.browse.save_name_box;
+        self.network_container.breadcrumb = &mut self.network.breadcrumb;
+        self.network_container.graph = &mut self.network.graph;
+
         self.ui_context.clear_hierarchy();
         self.browse.save_name_box.prepare_text(&mut self.font_system);
         self.browse.list_box.prepare_text(&mut self.font_system);
@@ -913,7 +923,10 @@ impl Application for FilesystemApp {
         // Start initial directory loading via FsService
         app.fs_service.send(services::fs::FsRequest::ReadLastDir);
 
-        app.rebuild_layout();
+        // NOTE: do not call rebuild_layout() here. This value is moved out of new()
+        // into the engine, which changes its address; the container/splitter widgets
+        // capture raw self-pointers during rebuild, so the first rebuild must happen
+        // after the move (the engine triggers it on the first frame via needs_rebuild).
         app
     }