Compatibility: preserve key subclass contracts and operator names

[JJJ]

Released 3.0.1 subclasses could fail at declaration time when 3.1 changed several extension signatures. Code extending the released comparison-operator class names also failed to autoload after those classes moved under Operators\Comparisons.

This change retains aliases for all 18 released comparison-operator class names and removes newly added native return types from selected released extension methods. Regression tests cover those contracts.

Three 3.1 signature changes remain intentional:

• parse_args() is now an array helper with optional $defaults. Subclasses overriding the 3.0 construction hook must rename that override to consume_args() before upgrading. A remaining one-argument override causes a PHP declaration fatal.
• Seven Column::is_*() predicates now accept an optional type argument. Subclasses overriding them must add that optional parameter before upgrading, or PHP rejects the subclass declaration.
• Column::get_name_sql() now accepts an optional $cast argument directly. Subclasses overriding it must add that optional parameter before upgrading, or PHP rejects the subclass declaration.

The changelog documents these migrations. An audit of the known BerlinDB consumer repositories found no downstream overrides of these signature families; private consumers may still need to update.

Validation: local PHP 8.1 and 8.2 / WordPress 6.7 / MariaDB 10.2 full suites (1,792 tests, 4,028 assertions each), PHPStan level 8, PHPCS, and git diff --check. The latest get_name_sql() update passed the PHP 8.2 / MariaDB 10.2 full suite and local static checks.