design-b D3 prep: pin fiber-on-context nesting and the foreign-stack refusal

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

tests/asyncify/sched-context.spec.ts

@@ -22,6 +22,8 @@ const SCENARIOS = [
   'one_transition_in_flight',
   'registry_refusals',
   'deep_park_sizing',
+  'fiber_nests_in_context',
+  'foreign_stack_refused',
   'async_wake',
 ];
 
@@ -43,6 +45,7 @@ type Stats = {
   finished: number;
   transitions: number;
   refusals: number;
+  foreignStackRefusals: number;
   running: number;
   transitionInFlight: boolean;
   readyQueued: number;
@@ -141,6 +144,16 @@ test.describe('Design B D1 — scheduler contexts', () => {
     // unknown id). Zero would mean those scenarios stopped provoking.
     expect(stats.refusals, 'illegal operations were refused and counted').toBeGreaterThan(0);
 
+    // --- the D3 blocker, pinned -------------------------------------------
+    // A wait called from a fiber running ON TOP of a context (a KiCad tool
+    // coroutine opening a dialog) must be refused, not allowed to yield
+    // someone else's context — that would save the tool fiber's stack into the
+    // host context's fiber struct. Exactly one scenario provokes it.
+    expect(
+      stats.foreignStackRefusals,
+      'a yield from a foreign stack was refused rather than corrupting the host context',
+    ).toBe(1);
+
     // --- sizing evidence (doc 20 risk 1: derive, don't inherit) ------------
     // The high-water mark is what a future buffer size must be justified by.
     // Assert it is both real (>0 — the parks did save frames) and comfortably