chore: clear compiler warnings

Remove accidental dead code (unused imports, the superseded paste tool()
helper, a never-read TORIENT handles field, a duplicate let binding, an
unneeded mut) and mark intentional-but-unwired scaffolding with
allow(dead_code): the web-only save_to_bytes, the plugin loadable() probe,
the planned PICK_HIGHLIGHT colours, the DynFieldSpec label, and the layout
ribbon helpers. Native build is now warning-clean apart from the upstream
nom/quick-xml future-incompat notices.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/ribbon/mod.rs

@@ -180,11 +180,13 @@ impl Ribbon {
     }
 
     /// Returns the index of the Layout module in the modules list.
+    #[allow(dead_code)] // layout-tab helpers; not yet wired
     pub fn layout_module_index(&self) -> Option<usize> {
         self.modules.iter().position(|m| m.id() == "layout")
     }
 
     /// Returns true if the currently active tab is the Layout module.
+    #[allow(dead_code)]
     pub fn active_is_layout(&self) -> bool {
         self.modules
             .get(self.active)