ui: component ownership and resource cleanup

[noelwelsh]

Reactive values created in a component's body — computeds, effects, event handlers — are never disposed when the component goes away. Currently nothing removes components, so nothing leaks, but any future dynamic component tree needs this.

Agreed design direction (see notes/ui-architecture.md):

• Ownership is a framework concern, invisible to application code. The React capability implementation is the natural place to register everything created for disposal, since all creation already goes through it.
• Ownership forms a tree of per-component context objects, not a single owner like the Runtime.
• Disposal responsibility lies with subscribers: a WritableSignal needs no disposal, a Computed or Effect must unsubscribe from its sources.
• The single app-facing lifecycle verb is onCleanup, for external resources.

[noelwelsh]

Prior-art survey of ownership/disposal in fine-grained reactive systems (S.js, SolidJS, Vue's effectScope RFC, Angular's effect API). All four converged on the same design:

• Ownership is implicit collection into a scope captured at creation time, never manual registration by the app.
• Scopes form a tree; disposal cascades to children.
• One app-facing verb for external resources (onCleanup / onScopeDispose / S.js S.cleanup).
• Creating a reactive node outside any scope is an error or warning (Angular: effect() must be called in an injection context; S.js/Solid warn without a root).

This matches the direction in notes/ui-architecture.md. Angular's injection-context requirement is the runtime-checked version of our React capability rule — we enforce the same thing at compile time.

One consequence of our design worth noting: the hard part of the S.js/Solid owner tree is computations created during another computation's run, which become children disposed on every re-run of the parent. We forbid exactly that case (React is unavailable inside Observe scopes), so ownership only ever needs to be per-component: the owner can simply be the component context object that already provides the React capability, with no per-run disposal machinery.

The one prior-art feature with no obvious analogue here is Vue's detached: true scopes (a scope not collected by its parent, for lazily initialized shared state) — probably YAGNI until there's a shared-state/composable story.