Missing CAS object during replay fails the build instead of recompiling

What happened

During the production rebench of the snapshot-era cas-plugin (canary.20 against kura 0.15.2), a cold build resolved action-cache entries from a stale snapshot whose blobs had since been evicted by production CAS churn. Two demand loads hit those evicted objects; FETCH_OBJECT correctly reported not-found after exhausting every source (pending instructions, publish cache, snapshot node table, remote batch read) — and the build failed with exit 65:

error: CAS error: missing object '0~PrRJCIbHVmuMQ7X9VJNQyyyXAF2aNmUp4xYutmJtjISAGh9GD51YHwmJBDP7_mD1h8MxlKVwM9mdtwecQLEYdQ=='

Why this matters

The eviction window is not closable server-side: any presence gate vouches at a point in time, and a build takes minutes — the per-key path has the same exposure (manifest fetched at minute 0, blobs pulled at minute 8 after eviction). The design assumption was that a genuinely unavailable object degrades to a warning + recompile (Swift’s compilation-replay path does exactly that), with clang’s hard-fail mitigated by retained fetch instructions and the snapshot fallback. This failure shows a consumer — the error shape suggests swift-driver’s own CAS read rather than compilation replay — that treats the miss as fatal.

What needs doing

1. Attribute the fatal consumer precisely (which task type issues this load, and through which llcas entry point).
2. Decide the mitigation: ideally the plugin degrades a definitive FETCH_OBJECT miss into something the consumer can recover from (e.g. surfacing the miss at action-cache-get level so the key resolves as a miss and the task recomputes, rather than failing a load mid-task) — or, if the consumer is inherently strict, the served association must be invalidated before the consumer commits to replaying it.
3. The staleness that widened the window in this incident is tracked separately (observability shipping in #11780); this issue stands regardless, because eviction can race even a fresh snapshot or a per-key hit.

Reproduced on: canary.20 client, kura 0.15.2, production tuist namespace, 2026-07-12.

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