cache-stampede-reference

Pure-reference catalog of cache-stampede (thundering-herd) phenomena and mitigations: parallel misses on key expiry trigger simultaneous recomputation (often congestion-collapse), countered by three families - locking, external recomputation near-expiry, and probabilistic early expiration via XFetch (`(time() - delta * beta * log(rand(0,1))) >= expiry`). Use when designing cache-refresh strategy or diagnosing a stampede incident. This is the single failure-mode pattern; for the broader multi-tier pattern catalog use cache-coherence-patterns-reference, for the stale-while-revalidate / stale-if-error extensions use stale-while-revalidate-reference; a reference consumed by redis-cache-tests, not a runnable test.