Add reproduction for issue #15618: selectbox reverts with format_func + custom classes

[sfc-gh-lwilby-1]

Summary

Adds a reproduction app and detailed investigation notes for streamlit/streamlit#15618 under issues/gh-15618/ (app.py + NOTES.md).

Reproduction status

Bug confirmed and root-caused on current develop (Streamlit 1.58.0).

• Occurs for frozen dataclasses and plain classes (not just plain classes); value-based __hash__/__eq__ does not prevent it.
• NamedTuple is immune.
• Removing the dict lookup from format_func avoids it.

Root cause

The widget state is correct ("two"), but st.selectbox() returns the default option ("one") to the script. validate_and_sync_value_with_options validates the stored selection by calling the user's format_func on it inside a broad except Exception. The stored value is a deepcopy of an instance of the previous rerun's class object (the script redefines the class each run); a dataclass's class-gated __eq__ makes x[s] raise KeyError, which the broad except treats as "value not in options" and resets to index 0. NamedTuple uses value-based tuple.__eq__, so its lookup never raises.

See issues/gh-15618/NOTES.md for the full trace and a suggested fix direction.