feat: fine-grained migration step control (closes #86)

[croc100]

Summary

• upgrade_to(revision) — upgrade to a specific revision
• upgrade_one() — upgrade exactly one step (+1)
• downgrade_one() — downgrade exactly one step (-1)
• downgrade_to(revision) — downgrade to a specific revision
• current_revision() — return current Alembic revision (None if at base)

Closes #86. Achieves pytest-alembic parity for the data migration testing pattern:

def test_data_migration(mrt):
    mrt.upgrade_to("abc123")
    mrt.seed("users", [{"email": "legacy@example.com"}])
    mrt.upgrade_one()
    # assert transformed state

Test plan

• test_upgrade_to — upgrades to specific revision, not beyond
• test_upgrade_one — single-step advance from mid-chain
• test_downgrade_one — single-step rollback
• test_downgrade_to — rollback to specific revision
• test_current_revision — tracks current revision through steps
• test_step_control_data_migration_pattern — end-to-end: seed at mid-point, verify data survives further upgrade

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pytest_mrt/plugin.py	83.33%	1 Missing and 1 partial ⚠️

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