Stress GitHub fallback under concurrent graph resolution and thundering-herd failures #191

Independent resilience follow-up to zed-cli #388 and zed-clients negative-cache/deadline hardening.

Exercise a dependency graph with repeated/shared package identities while the primary Zed control plane is unavailable and selected GitHub probes fail. The test should prove bounded external work rather than only functional success.

Acceptance criteria:

• pin exact zed-cli and zed-clients candidate SHAs;
• construct a graph with shared transitive nodes and repeated source identities;
• run concurrent resolution/fetch with a clean HOME/store;
• instrument local fault-injection endpoints to count requests by source kind without collecting secrets;
• prove failed deterministic probes are deduplicated per resolution operation and distinct package identities remain independent;
• assert one end-to-end deadline bounds the operation under many failing candidates;
• verify successful immutable artifacts are stored once and all consumers reuse them;
• repeat on Linux plus at least one non-Linux runner; retain sanitized request-count/timing evidence.