[ty] Avoid quadratic analysis of repeated statement calls (#26775)
## Summary
Prior to this change, each statement-level call immediately applied an
`IsNonTerminalCall` narrowing gate to every live place so calls
returning `Never` could invalidate later narrowing. Repeated calls
therefore rebuilt all live place states after every statement, which is
quadratic.
We now store the call narrowing gate alongside the pending scope-wide
reachability constraint. Reachability continues to use the regular
materialization path at every use, while narrowing gates are replayed
only before a place changes or merges. A `Never`-returning call
therefore still gates narrowing later on the same branch without eagerly
rewriting every live place state.
Closes https://github.com/astral-sh/ty/issues/3986.
