[ty] Narrow walrus expression values (#26949)
## Summary
Fixes astral-sh/ty#4020.
Narrowing predicates on the right-hand side of a walrus expression were
evaluated only as simple expressions, so checks such as `if result :=
isinstance(value, int):`, `TypeGuard`/`TypeIs` predicates, comparisons,
and aliased conditions failed to narrow `value`.
This recursively evaluates the walrus value as a predicate, preserves
the target's truthiness constraint, and discards constraints on the
rebound target and its member places.
## Test plan
Added focused mdtests for `isinstance`, `TypeGuard`/`TypeIs`,
equality/identity comparisons, and aliased predicates.
