We play weird ownership games with SignalTextChanged, and we don't know why

Nobody has claimed this yet.

See context: #13876, [MSFT:39994969](https://task.ms/39994969)

See discussion:
> I'm not really in favor of this PR. There shouldn't be situation to begin with were NVDA calls us and the existence of `TermControl` is the make/break of whether WT crashes... As such I'd prefer fixing whatever UIA APIs we have that might crash WT, including any assumptions we have that WinUI won't suddenly call `AppHost::~AppHost` despite being 50 calls deep in the stack.

> > Should we file a workitem to fix this more soundly? Should we take this as a "stopgap" until we can fix it more soundly?
>
> Maybe I was too terse. 😄 I'm in favor of merging this, just not in favor of this PR because it's a stopgap. I think we should just continue to work on cleaning up our pointer safety and data sharing behavior. Maybe even do it consciously at some point instead of doing it on a case by case basis. IMO we should remove any use of `this` (including implicit use) if it can be avoided and find and if possible remove implicit recursion like this one were class X calls Y which calls back into X (e.g. introduce class Z which is used by both X and Y, removing the need for recursion).

> > Maybe even do it consciously at some point instead of doing it on a case by case basis.
>
> YES

Start with issue #13876 and the linked MSFT work item, then trace SignalTextChanged, TermControl, and the UIA calls discussed here. The work is done when the ownership, pointer-safety, and data-sharing behavior no longer depend on the described stopgap or implicit recursive calls.

Written by the indexing model from the issue text.