Add REPLACE(str, from, to) scalar function #67

Replace all occurrences of a substring. REPLACE(phone, '-', '').

Scope: follow the existing scalar functions in src/scalar.zig (see UPPER/TRIM). Parse in src/parser.zig, evaluate per-row into the scalar arena.

Done when: works in SELECT, three string args, empty to deletes, tested. std.mem.replace does the heavy lifting.