Add LPAD / RPAD string functions #160

SELECT LPAD(col, n, 'x'), RPAD(col, n, 'x') — pad a string column to length n with a fill character (defaulting to space if the third arg is omitted, matching most SQL dialects). Not currently supported.

Closest existing pattern: SUBSTR(col, start, len) in src/scalar.zig for a multi-arg string function with a numeric arg. Two new functions but they share almost all their logic (just pad-left vs pad-right), so one PR covering both is reasonable.

Add to the SQL Reference table in README.md and unit tests for both, including the omitted-third-arg default.