Working on https://github.com/tursodatabase/turso/issues/2964 I came
upon `walk_expr_mut`, I don't think it existed last time I really spent
much time in the translator. So quickly went back and cleaned this up.
The MakeRecord instruction now accepts an optional affinity_str
parameter that applies column-specific type conversions before creating
records. When provided, the affinity string is applied
character-by-character to each register using the existing
apply_affinity_char() function, matching SQLite's behavior.
Fixes#2040Fixes#2041