test(bench): split async producer attribution probes

[LimiNode]

Summary

• add focused probes for task-object construction and payload ownership
• separate prebuilt TaskExecutor admission from full payload queue admission
• measure Logger dispatch with task construction but without enqueueing
• document overlapping attribution signals and preserve existing full-path cases

Verification

• MSVC Release C++17 MPSC build and 100k/10k/5 run
• MSVC Release C++17 mutex-deque build and smoke run
• production implementation unchanged