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Remove accidental dead code (unused imports, the superseded paste tool()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/io/mod.rs

@@ -361,6 +361,7 @@ pub fn save(doc: &CadDocument, path: &Path) -> Result<(), String> {
 /// `doc.version`). Used by the web build, which hands the bytes to a browser
 /// download instead of writing a path — the byte-buffer counterpart to
 /// [`save_as_version`].
+#[allow(dead_code)] // web/wasm build only; unused in the native target
 pub fn save_to_bytes(
     doc: &CadDocument,
     ext: &str,