[ty] Track narrowing unions to prevent exponential blowup
When inferring a call expression with a union type context, we try narrowing
to each element of the union. If nested calls have the same union as their
parameter type, this would lead to exponential blowup. By tracking which
unions we're already narrowing against, we skip redundant nested narrowing.
On a synthetic benchmark with nested `list_schema()` calls:
- 4-level nesting: >120s → 0.13s
