Fix SCXML state action ordering

[JovaniPink]

Summary

• execute state entry actions in parser document order
• execute state exit actions in reverse document order
• remove the two unresolved ordering TODOs from the SCXML algorithm core
• add observable regressions for nested transitions and parallel regions declared in both orders
• record the correctness fix in the existing untagged 0.7.0 changelog entry

Why

The entry and exit sets are intentionally represented as sets while they are computed. Iterating those sets directly made side-effect ordering depend on Python object iteration rather than statechart document order.

SCXML requires deterministic ordering: ancestors and earlier document states enter first, while descendants and later document states exit first. This is observable whenever states declare entry or exit actions, especially across parallel regions.

Behavior covered

• nested external transition: child exit, parent exit, transition action, target entry
• parallel entry: enclosing state, first declared region and leaf, then second declared region and leaf
• parallel exit: second declared region and leaf exit before the first, then enclosing state
• both parallel region declaration orders produce the corresponding deterministic trace

Scope

This changes only state action ordering inside the existing microstep algorithm. Transition selection, transition domains, context updates, handler adaptation, snapshots, and public APIs are unchanged.

Validation

• poetry run python -m pytest tests/test_action_ordering.py -q (5 passed)
• poetry run python -m pytest tests/ --ignore=tests/test_scxml.py (402 passed)
• poetry run python -m pytest tests/test_scxml.py (54 passed)
• poetry run mypy src/xstate/
• poetry run ruff format --check src/ tests/ docs/examples/
• poetry run ruff check src/ tests/ docs/examples/
• git diff --check

[gemini-code-assist]

Code Review

This pull request ensures that state entry actions run in document order and exit actions run in reverse document order, including nested and parallel configurations. This is achieved by sorting the states to enter and exit by their order attribute in src/xstate/algorithm.py. Additionally, a new test file tests/test_action_ordering.py has been added to verify these ordering behaviors. No review comments were provided, and the implementation is clean and well-tested.

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