Context varargs can smuggle graph-runtime references past the borrow tree's lifetime discipline

[Keavon]

Reported by @TrueDoctor following the rank polymorphism PR (#4335). Three existing mechanisms combine into an unsound API surface:

1. The borrow tree launders lifetimes: NodeContainer derefs *const TypeErasedNode<'static> to &'static, so references handed out inside the graph (the &PlatformEditorApi scope wire, &WgpuExecutor, or any reference to node-owned state) are typed 'static but actually live only until the next graph recompilation deallocates their owner.
2. OwnedContextImpl::with_vararg accepts any Box<dyn AnyHash + Send + Sync>. Its 'static bound cannot distinguish real ownership from a laundered reference, so safe code in any node can store such a reference into a context. Make the data model use Item and List types universally, with nodes authored as rank-polymorphic kernels #4335 made this channel idiomatic (render boundary config, per-item lambda rows).
3. Contexts escape the graph: the Monitor node snapshots IORecord { input: Context, output } into an Arc<dyn Any> that the editor introspects and can retain across recompiles.

Combined: a node stores a laundered reference as a vararg, a Monitor captures the context, the graph recompiles and frees the referent, and the editor reads freed memory through entirely safe code (vararg() + downcast_ref). A vararg with interior mutability would similarly let context readers mutate graph-owned state as a covert return channel.

All current with_vararg callers box owned data, so no UB exists in the code today. But the risk remains for the API permitting it silently.