[ty] TypedDict: Add support for typing.ReadOnly (#20241)

## Summary

Add support for `typing.ReadOnly` as a type qualifier to mark
`TypedDict` fields as being read-only. If you try to mutate them, you
get a new diagnostic:

<img width="787" height="234" alt="image"
src="https://github.com/user-attachments/assets/f62fddf9-4961-4bcd-ad1c-747043ebe5ff"
/>


## Test Plan

* New Markdown tests
* The typing conformance changes are all correct. There are some false
negatives, but those are related to the missing support for the
functional form of `TypedDict`, or to overriding of fields via
inheritance. Both of these topics are tracked in
https://github.com/astral-sh/ty/issues/154

crates/ty_python_semantic/src/place.rs

@@ -589,6 +589,11 @@ impl<'db> PlaceAndQualifiers<'db> {
         self.qualifiers.contains(TypeQualifiers::NOT_REQUIRED)
     }
 
+    /// Returns `true` if the place has a `ReadOnly` type qualifier.
+    pub(crate) fn is_read_only(&self) -> bool {
+        self.qualifiers.contains(TypeQualifiers::READ_ONLY)
+    }
+
     /// Returns `Some(…)` if the place is qualified with `typing.Final` without a specified type.
     pub(crate) fn is_bare_final(&self) -> Option<TypeQualifiers> {
         match self {