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Remove accidental dead code (unused imports, the superseded paste tool()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 <[email protected]>

src/plugin/external.rs

@@ -47,6 +47,7 @@ impl ExternalPlugin {
 
     /// True when the package can be loaded today: compatible API *and* a native
     /// library present for this platform.
+    #[allow(dead_code)] // plugin-host surface (issue #100); not yet wired
     pub fn loadable(&self) -> bool {
         self.api_compatible() && self.lib_present
     }