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/project.rs

@@ -17,7 +17,7 @@
 //! Project: owns the graph, the folder tree, settings, and the undo
 //! stack binding. Mirrors C++ `olive::Project`.
 
-use std::sync::{Arc, Mutex, Weak};
+use std::sync::{Arc, Mutex};
 
 use crate::graph::Graph;
 use crate::id::NodeId;
@@ -451,7 +451,3 @@ pub enum ChangeRecord {
 	},
 }
 
-/// Weak-project handle used by the identity registry (node_from_identity
-/// upgrades it; a freed project leaves a dead weak entry that upgrades
-/// to `None`).
-pub(crate) type WeakProject = Weak<Mutex<Project>>;