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-codec/src/proxymanager.rs

@@ -97,6 +97,9 @@ impl ProxyParams {
 		cstr_slice(&self.extension)
 	}
 
+	/// Only the proxy-params tests read the preset through this accessor
+	/// (the struct field itself is `pub`).
+	#[allow(dead_code)]
 	fn preset_str(&self) -> &str {
 		cstr_slice(&self.preset)
 	}