feat: v1.6.0 — step control, Django mode guards, timeout recovery

[croc100]

Summary

• Fine-grained migration step control (upgrade_to, upgrade_one, downgrade_one, downgrade_to, current_revision) — closes feat: fine-grained migration step control in mrt fixture (pytest-alembic parity) #86
• Bug: Django mode step methods raised AttributeError — now raises RuntimeError with clear message
• Bug: check_static() raised AttributeError in Django mode — now raises RuntimeError
• Bug: timeout path skipped DB state recovery — mirrors except Exception recovery path
• Tests: 9 new tests covering all three bug fixes
• All docs updated: README, CHANGELOG, ROADMAP, docs/api.md, docs/cli.md, docs/index.md, docs/migration-guide.md, docs/roadmap.md
• Version bump: 1.5.0 → 1.6.0

Test plan

• 463 tests pass, 18 skipped (all existing + 9 new)
• Django mode upgrade/downgrade family raises RuntimeError (7 parametrized cases)
• Django mode check_static() raises RuntimeError
• Timeout triggers DB state recovery (downgrade_base called)

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pytest_mrt/core/verifier.py	50.00%	3 Missing and 1 partial ⚠️

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