Emit `os_signpost` with spans

[armcknight]

This way, local profiling with Xcode Instruments gets them for free, to help debug issues seen in the Sentry dashboard and validate fixes.

[github-actions[bot]]

This issue has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you label it Status: Backlog or Status: In Progress, I will leave it alone ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀