fix(proxy-state-tree): add missing stopTracking() method to TrackStateTree

[ericsorenson]

Summary

• Add missing stopTracking() method to TrackStateTree class and ITrackStateTree interface in proxy-state-tree
• Vue (v2, v3) and Svelte integrations call tree.stopTracking() on every component update, but the method didn't exist, causing a TypeError at runtime
• The fix delegates to this.root.unsetTrackStateTree(this), mirroring how subscribe() already stops tracking internally
• Add unit test verifying state accesses after stopTracking() are not recorded

The React integration skips stopTracking() entirely. It goes straight from track() to subscribe(), and subscribe() already calls this.root.unsetTrackStateTree(this) as its first line. So React never needed a separate stopTracking(), untracking happens implicitly when it subscribes. Vue and Svelte have a different render lifecycle where they call stopTracking() explicitly between tracking and subscribing, which is why only those integrations hit the TypeError.

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[ericsorenson]

After a conversation, we decided to stop using stopTracking and follow the pattern used for React.