fix: support mixed ASC/DESC directions in ORDER BY

[SebTardif]

Summary

Fixes #3. ORDER BY with mixed ASC/DESC directions (e.g. ORDER BY name ASC, age DESC) now correctly applies each column's direction independently.

Root Cause

_execute_order_by used a single reverse flag for the entire sorted() call, determined by the first ORDER BY column's direction. All columns were sorted in the same direction.

Fix

Replaced the single-flag approach with a per-column comparator using functools.cmp_to_key. Each ORDER BY column now respects its own ASC/DESC direction. NULL handling is preserved (NULLs sort last regardless of direction).

Test

Added test_order_by_mixed_directions that sorts by active DESC, age ASC and verifies both the group ordering and within-group ordering are correct.

Verification

• 96 tests pass
• ruff lint and format clean