Expose completion as DSL event token

[gabewillen]

Summary

• expose sml::completion<T> as a first-class DSL token so transitions can be written as completion<T>
• keep completion-event internals unchanged while removing the need to wrap with event<...> at call sites
• update affected functional tests to validate direct completion<T> usage

Testing

• ./scripts/quality_gates.sh
• cmake --build build --target test_transitions test_actions_process_n_defer -j8 && ./build/test/ft/test_transitions && ./build/test/ft/test_actions_process_n_defer

[Copilot]

Pull request overview

This PR exposes sml::completion<T> as a first-class DSL event token, allowing it to be used directly in transition tables without wrapping it in event<...>. The implementation follows the same pattern as other internal event tokens like on_entry, on_exit, unexpected_event, and exception.

Changes:

• Modified completion from a type alias to a variable template wrapped in front::event<back::completion<TEvent>>
• Updated functional tests to use the simplified completion<T> syntax instead of event<completion<T>>

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
include/boost/sml.hpp	Changed completion from type alias to variable template matching the pattern of other DSL event tokens (on_entry, on_exit, etc.)
test/ft/transitions.cpp	Updated test to use simplified completion<e1> syntax instead of event<completion<e1>>
test/ft/actions_process_n_defer.cpp	Updated tests to use simplified completion<T> syntax in multiple transition table definitions

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