Fiber isolation

Understudy keeps one context per fiber, not one static bag per process.

This is the canonical page for the behaviour. The Testo adapter describes what it does about it and links here.

What the split buys

Each fiber gets its own recording phase, call log and sequence counter. Two fibers exercising the same double do not interleave into one sequence, so a protocol claim made in one is not broken by traffic from the other.

What stays whole

Isolation and accounting are different things:

	Per fiber	Across every context the test used
Recording phase, call log, sequence counter	yes
verifyAll(), reset(), idle(), checkpoint()		yes

A body that runs in a fiber is still the test's, and an adapter asks about the test from wherever it stands.

The failure this closes

Without it, a test body run inside a fiber could leave an unmet expect() behind while the suite stayed green — a false pass, which is the one failure mode a verification library must not have.

It is not free: the accounting that makes it work costs roughly 0.3µs on every double built, and that trade is recorded with its numbers in Performance. The trade was reviewed and accepted rather than absorbed quietly.

The rule to remember

Configuration and verification must run in the context that owns the double. Ordinary calls may be made from another fiber and are still recorded in the owner's log.

A double created inside Understudy::scope() is invalid after that scope closes — the same rule, with the nesting made explicit.