[ty] Improve disjointness inference for NominalInstanceTypes and SubclassOfTypes (#18864)

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

crates/ty_python_semantic/resources/mdtest/annotations/callable.md

@@ -192,16 +192,18 @@ def _(
 from typing import Callable, Union
 from ty_extensions import Intersection, Not
 
+class Foo: ...
+
 def _(
     c: Intersection[Callable[[Union[int, str]], int], int],
     d: Intersection[int, Callable[[Union[int, str]], int]],
-    e: Intersection[int, Callable[[Union[int, str]], int], str],
-    f: Intersection[Not[Callable[[int, str], Intersection[int, str]]]],
+    e: Intersection[int, Callable[[Union[int, str]], int], Foo],
+    f: Intersection[Not[Callable[[int, str], Intersection[int, Foo]]]],
 ):
     reveal_type(c)  # revealed: ((int | str, /) -> int) & int
     reveal_type(d)  # revealed: int & ((int | str, /) -> int)
-    reveal_type(e)  # revealed: int & ((int | str, /) -> int) & str
-    reveal_type(f)  # revealed: ~((int, str, /) -> int & str)
+    reveal_type(e)  # revealed: int & ((int | str, /) -> int) & Foo
+    reveal_type(f)  # revealed: ~((int, str, /) -> int & Foo)
 ```
 
 ## Nested

crates/ty_python_semantic/resources/mdtest/annotations/int_float_complex.md

@@ -88,3 +88,26 @@ def assigns_complex(x: complex):
 def f(x: complex):
     reveal_type(x)  # revealed: int | float | complex
 ```
+
+## Narrowing
+
+`int`, `float` and `complex` are all disjoint, which means that the union `int | float` can easily
+be narrowed to `int` or `float`:
+
+```py
+from typing_extensions import assert_type
+from ty_extensions import JustFloat
+
+def f(x: complex):
+    reveal_type(x)  # revealed: int | float | complex
+
+    if isinstance(x, int):
+        reveal_type(x)  # revealed: int
+    elif isinstance(x, float):
+        reveal_type(x)  # revealed: float
+    else:
+        reveal_type(x)  # revealed: complex
+
+    assert isinstance(x, float)
+    assert_type(x, JustFloat)
+```