Summary

• raise CoverageException if switch_context() is called while running on core=sysmon
• keep the existing startup-time fallback for configured dynamic contexts unchanged
• document that explicit context switching requires ctrace or pytrace, and add a regression test

Problem

switch_context() worked under ctrace, but under sysmon a shared line hit by two contexts only kept the first one. The API path had no guard, so Python 3.14's default sysmon core could silently produce wrong context data.

Solution

Instead of silently downgrading to a slower tracing core after measurement has already started, reject the API call and tell users to choose core=ctrace or core=pytrace explicitly when they need dynamic contexts.