Add completion<T> post-event transition support

[gabewillen]

Summary

• add a new internal event type completion<T> and a public alias sml::completion<T>
• run completion<origin_event_type> after each handled event (before anonymous progression)
• apply the same post-event behavior for queued/deferred events, refreshing origin type per popped event
• add functional tests for completion ordering and queue/defer origin refresh behavior

Details

• process_event now routes through a shared helper that does:
  1. normal dispatch for the incoming event
  2. post-event loop: completion<origin> first, then anonymous transitions
• process_event_no_defer and process_event_no_queue use the same helper so semantics are consistent

Tests

• ./scripts/quality_gates.sh
  • format: pass
  • clang-tidy: pass
  • regression matrix: pass
  • sanitizer matrix: pass
  • coverage gate: pass (99.5% >= 90%)
  • benchmark smoke build: pass

Bench snapshots

• compared before/after snapshots locally; no meaningful runtime regression observed for the sampled benches

[Copilot]

Pull request overview

This PR adds support for a new internal event type completion<T> that allows transitions to fire after a specific event type has been handled but before anonymous transitions. The completion events maintain the origin event type even when events are processed from queues or defer queues.

Changes:

• Added completion<T> internal event type and public API alias
• Refactored event processing to run completion events before anonymous transitions in a unified post-event loop
• Added comprehensive tests for completion event ordering and queue/defer origin tracking

Reviewed changes

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

Show a summary per file

File	Description
include/boost/sml.hpp	Core implementation: added completion event type, event mapping helpers, and refactored process_event to use process_event_and_post_events with unified post-event processing loop
test/ft/transitions.cpp	Added test verifying completion events run before anonymous transitions
test/ft/actions_process_n_defer.cpp	Added tests for completion events with process queue and defer queue, verifying origin type is refreshed per popped event; also fixed missing trailing newline
test/ft/issue_171.cpp	Added regression test ensuring wildcard event handlers are not triggered by internal completion/anonymous events
test/ft/CMakeLists.txt	Registered new issue_171 test

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