Support Test::Unit lifecycle callbacks

[nertzy]

Test::Unit users cannot currently register with_model through Test::Unit’s native fixture callbacks. This layer adds :test_unit to the existing lifecycle dispatch rather than introducing a separate implementation.

• Create models before inherited and user setup callbacks
• Destroy models in reverse order after inherited and user teardown callbacks
• Preserve original setup and teardown errors during cleanup
• Add isolated lifecycle and STI parity coverage

Part of #68. Stack layer 2 of 4.