gh-144513: Skip critical section locking during stop-the-world

[colesbury]

When the interpreter is in a stop-the-world pause, critical sections don't need to acquire locks since no other threads can be running. This avoids a potential deadlock where lock fairness hands off ownership to a thread that has already suspended for stop-the-world.

• Issue: Improve critical sections and stop-the-world interoperability #144513

[colesbury]

A few notes:

• Mostly written by Claude code
• Test passes on PR and would deadlock on main without the associated fix
• Part of the motivation is making frame locals (framelocalsproxy) access thread-safe. Some of the time we will want to use a STW pause, and allowing the existing critical sections within that pause will make the modifications simpler