Detect changed Check constraint expressions in migrations

[ebrake]

Closes #39

Fixes makemigrations not detecting changes to an existing Check constraint when the constraint name stays the same.

Before this change, the migration diff only compared check constraints by name, so this produced no migration:

Check("age >= 0", name="chk_users_age")

when changed to:

Check("age > 0", name="chk_users_age")

Now the diff detects same-name expression changes and emits:

1. drop_check for the old constraint
2. add_check for the new constraint

The drop happens first because the replacement uses the same constraint name.

Reordered logic in crates/oxyde-migrate/src/diff.rs so changed same-name checks can be handled as drop_check then add_check.

Testing

cargo test -p oxyde-migrate --quiet
cargo test --workspace --exclude oxyde-core-py --quiet
uv run --project python --extra dev --no-sync pytest -q python/oxyde/tests/unit/test_migrations_execution.py