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-core/src/debug.rs

@@ -130,16 +130,6 @@ impl Level {
 			Level::Error | Level::Fatal => log::Level::Error,
 		}
 	}
-
-	/// From a facade filter level (for [`log_get_level`]).
-	fn from_filter(f: LevelFilter) -> Level {
-		match f {
-			LevelFilter::Off | LevelFilter::Error => Level::Error,
-			LevelFilter::Warn => Level::Warning,
-			LevelFilter::Info => Level::Info,
-			LevelFilter::Debug | LevelFilter::Trace => Level::Debug,
-		}
-	}
 }
 
 /// Oak-level name for a facade level (the sink path). `log::Level` has