workspace: zero compiler warnings across all targets

254 warnings (320 counting replayed-cache re-emitters) cleaned:
unused mut/imports/variables, irrefutable if-lets and unreachable
patterns, dead code removed or annotated #[allow(dead_code)] with
the reason (C++ parity value sets, cfg(test) helpers, public API
reservations), drop(&ref) no-ops removed, fn-pointer identity via
std::ptr::fn_addr_eq, the test-stubs feature declared in
oak-node's manifest, missing docs filled. Every unused-Result site
was judged individually: meaningful errors propagate, intentional
ignores are let _ = with a note.

Two pre-existing latent bugs are documented in place, behavior
preserved: app.rs's timeline-tool observer and dialogs.rs's format
subscription both drop the returned Subscription immediately, so
they never fire.

crates/oak-node/src/graph.rs

@@ -664,7 +664,6 @@ impl Graph {
 		if index < 0 || index > size as i32 {
 			return Err(Error::Invalid);
 		}
-		drop(input_);
 		let entry = self.get_mut(id).ok_or(Error::NotFound)?;
 		entry.core.input_array_insert(input, index as usize);
 
@@ -703,7 +702,6 @@ impl Graph {
 		if index < 0 || index >= size as i32 {
 			return Err(Error::Invalid);
 		}
-		drop(input_);
 		let entry = self.get_mut(id).ok_or(Error::NotFound)?;
 		entry.core.input_array_remove(input, index as usize);