Add investigation notes and root cause for issue #15618

[sfc-gh-lwilby-1]

Follow-up to #62 (which was already merged before these changes were pushed).

Adds issues/gh-15618/NOTES.md with the full root-cause analysis and refines the workaround explanation in issues/gh-15618/app.py.

Root cause (confirmed)

The selectbox 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 is immune because it uses value-based tuple.__eq__.

Verified via a headless AppTest matrix and a monkeypatched trace of the validation helper. See NOTES.md for details and a suggested fix direction.