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-task/src/render.rs

@@ -1051,9 +1051,9 @@ impl RenderTask {
 		unsafe {
 			drop(Box::from_raw(dispatch));
 		}
-		// The pump closure borrows `private_dispatch`; drop it before the
-		// take below.
-		drop(pump);
+		// `pump`'s shared borrow of `private_dispatch` ends at its last use
+		// above (NLL); the closure needs no explicit drop, which would be a
+		// no-op because a closure capture is `Copy` here.
 		if let Some(d) = private_dispatch.take() {
 			d.shutdown();
 		}