perf(macos): expose frame scheduler observability - #412

Summary

• add effect-completion batches to frame-profile stage measurements
• emit scheduler deadline, queue lateness, producer, and dispatch timing when GPU frame tracing is enabled
• expose the new stage through automation snapshots and benchmark output
• document the observable profile surface

This adds diagnostics only; it deliberately does not claim or alter the scheduler root cause. The branch is rebased onto current main and was independently reviewed before publication.