fix(bindx): drop vestigial handle dispose lifecycle that broke late writes

[matej21]

Problem

Handles are stateless live views over the store: the only subscriptions they create are returned from subscribe() and owned by useSyncExternalStore, not stored on the handle. So dispose() freed nothing — it only set _disposed and armed assertNotDisposed() to throw on legitimate late writes through an accessor a consumer still holds (e.g. the Slate block editor dispatching setValue from an async onChange after the entity version bumped and EntityHandleRenderer recreated the handle).

The dispose machinery was orphaned by the migration away from the eager identityMap subscription model, where handles really did hold _unsubscribe.

Change

• Remove the dispose lifecycle from handles: dispose/isDisposed/assertNotDisposed in BaseHandle, the dispose() overrides, all assertNotDisposed() call sites, and dispose() from the internal RelationHandleRaw interface.
• Delete the now-dead core/Disposable.ts.
• Stop disposing superseded handles in useEntity and <Entity>.
• reset() stays — it is a real operation.

Tests

Adds a regression test covering a captured field accessor that stays writable across a data-driven re-render.

Split out of #54 — this fix was previously bundled with the issue-#53 orderBy fix; the two are independent and now have separate PRs.

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