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Pull #2821

github

Add Layer-2 concurrency stress specs

Three targeted tests firing barrier-released concurrent requests at one
shared compiled API, each asserting an exact thread-specific response so
cross-request contamination fails deterministically rather than being
observed by luck:

- Array index tracking isolation: each thread plants the invalid element
  at its own index and must get exactly its own index back in the error
  message — pins ParamScopeTracker's fiber-local per-request state
  (teeth-verified: a corrupted store_index fails the spec).

- Mutable default isolation: each request must see exactly its own
  mutation of a default: { tags: [] } param. Pins the end-to-end
  property enforced jointly by DefaultValidator's per-request dup and
  the params container's copy-on-assignment.

- Cold-boot contention: fresh uncompiled apps hit by racing first
  requests, exercising Instance#compile! (double-checked LOCK) and the
  first traversal of the shared coercer graph.

On MRI these catch logic bleed; the edge workflow runs the full suite on
jruby-head/truffleruby-head, so they also run under true parallelism
there (Layer 3) with no extra wiring.

Test-only; no production changes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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