[ty] more precise lazy scope place lookup (#19932)

## Summary

This is a follow-up to https://github.com/astral-sh/ruff/pull/19321.

Now lazy snapshots are updated to take into account new bindings on
every symbol reassignment.

```python
def outer(x: A | None):
    if x is None:
        x = A()

    reveal_type(x)  # revealed: A

    def inner() -> None:
        # lazy snapshot: {x: A}
        reveal_type(x)  # revealed: A
    inner()

def outer() -> None:
    x = None

    x = 1

    def inner() -> None:
        # lazy snapshot: {x: Literal[1]} -> {x: Literal[1, 2]}
        reveal_type(x)  # revealed: Literal[1, 2]
    inner()

    x = 2
```

Closes astral-sh/ty#559.

## Test Plan

Some TODOs in `public_types.md` now work properly.

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/ty_python_semantic/resources/mdtest/narrow/conditionals/nested.md

@@ -248,6 +248,13 @@ def f(x: str | None):
                 reveal_type(x)  # revealed: str | None
     x = None
 
+def f(x: str | None):
+    def _(x: str | None):
+        if x is not None:
+            def closure():
+                reveal_type(x)  # revealed: str
+    x = None
+
 def f(x: str | None):
     class C:
         def _():
@@ -303,13 +310,17 @@ no longer valid in the inner lazy scope.
 def f(l: list[str | None]):
     if l[0] is not None:
         def _():
-            reveal_type(l[0])  # revealed: str | None
+            # TODO: should be `str | None`
+            reveal_type(l[0])  # revealed: str | None | @Todo(list literal element type)
+        # TODO: should be of type `list[None]`
         l = [None]
 
 def f(l: list[str | None]):
     l[0] = "a"
     def _():
-        reveal_type(l[0])  # revealed: str | None
+        # TODO: should be `str | None`
+        reveal_type(l[0])  # revealed: str | None | @Todo(list literal element type)
+    # TODO: should be of type `list[None]`
     l = [None]
 
 def f(l: list[str | None]):