Two scenarios, deliberately separate because they answer different
questions that D3 depends on:
- fiber_nests_in_context: can a libcontext-style fiber run nested inside
a scheduler context at all? YES. Worth pinning explicitly — an earlier
version of this scenario trapped `unreachable` and briefly looked like
proof that nesting was impossible, which would have been a serious
wrong turn for the plan. The real cause was the test's own bug: the
host-side emscripten_fiber_t was zero-initialised, so the swap unwound
into a null asyncify buffer. Nesting works; the scenario now proves it.
- foreign_stack_refused: a yield from that nested fiber is REFUSED, and
the host context and scheduler both survive it.
The spec asserts foreignStackRefusals == 1, so the refusal path cannot
silently stop being exercised.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TEHGiiXMShNXbBr7gSJ7iz
6e80827cec