Expression WARNING diagnostics (type mismatch, = NULL) carried
a coarse span — the whole WHERE clause, from the `where`
keyword to end of input. They now span exactly the offending
literal operand, read from the Operand source span added in the
previous commit. predicate_warnings derives the span per
warning; pair_type_mismatch returns (message, literal-span);
the dead where_clause_span helper is removed.
5 walker tests assert the spans cover exactly the literal /
identifier (type mismatch, = NULL, BETWEEN bounds, IN item,
unknown-column ERROR). 1105 passing, clippy clean.
claude@clouddev1
3912fb5a9b