Tests: lock in the ``LDR_TEST_MODE`` boolean-parsing behaviour for the SQLCipher KDF floor. Adds direct ``_get_min_kdf_iterations()`` unit tests for falsey values (``0``/``false``/``no``/``off`` must NOT relax the floor — the #4564 regression) and the full truthy set (``on``/``enabled`` must relax it, guarding against a narrower parser). Also moves the integration test off the registry's ``min_value`` boundary so it exercises the KDF-floor clamp rather than range validation.
