Summary

Fixes a cache-invalidation bug for SignalingList. See #3481

Motivation

Because mutable lists mutate in-place, the reactive graph's current_val != old_val check compares the list's memory address against itself. It always evaluates to False, causing the @computed_property to return a stale cache. Furthermore, storing the actual list object in ComputedState traps massive data structures in memory.

Implementation

Passed None to the dependency tracker whenever the accessed value is a MutableSequence. This keeps the list out of the ComputedState and ensures in-place mutations successfully bypass the old != new cache check. Immutable standard observables remain unaffected.

Closes #3481