[lucataco]

Follow-up to #232, which is merged. This PR is one commit on main.

Why

After #232, a fill that began before the source reported a key gone can finish after the eviction and write the old value back. The value lasts until the next refresh, at most the fresh TTL, because that refresh evicts it again.

What changed

• A random generation per eviction. Each eviction replaces the key's generation, cache:gen:<name>:<key>, with a random value in the same script that deletes the data and freshness keys.
• Compare before writing. A fill reads the generation before it fetches and writes through a compare-and-set script only if the generation is unchanged. A shadow write compares against the shadow's own generation.
• Limits. A fill older than 2 minutes is not written. Generations expire after 4 minutes, which outlives any fill that could still be written.
• Observability. A skipped fill sets cache.fill_discarded on its span to generation_changed, generation_read_failed, or max_age.

Cost

• Every fill on every cache changes. Each fill adds one GET per backend, and the write becomes an EVAL instead of a MULTI pipeline.
• Each eviction runs an EVAL per backend.
• Each evicted key leaves one generation key. It has a 26-byte value and a 4-minute TTL.
• Redis topology. The scripts touch several keys of one entry, so they need standalone or Sentinel Redis, not Redis Cluster.

Keep this a draft until the extra Redis work per fill has been measured. Then decide whether closing the late-fill window is worth that cost.

Verification

• Tests that failed before their fixes:
  • a late fill after an eviction
  • a late fill after the generation key expires
  • the generation TTL bound
  • the discard span attribute
• The tests from fix(cache): stop serving a value after the source reports it gone #232 are kept.
• Package checks: go test -race -shuffle=on ./cache passes, and golangci-lint v2.1.5 reports no issues.
• End to end against the API service with this code, on a local Compose stack: after the source reported a key gone, the API never served the key again. Redis showed a random generation with a TTL just under 4 minutes.