Implement stale-while-revalidate strategy in OptimisticCache

[Kingsman-99]

Description

OptimisticCache returns cached values until TTL expiry and then blocks on a fresh fetch. Under stale-while-revalidate, the stale value is returned immediately while a background refresh is triggered, eliminating latency spikes at TTL boundaries.

Acceptance Criteria

• OptimisticCacheOptions gains staleWhileRevalidateMs (default 0, meaning disabled)
• When a cached value is within staleWhileRevalidateMs of its TTL, it is returned immediately and a background revalidation is started
• Background errors do not propagate to the caller; they are emitted as a revalidateError event
• Unit tests pass

Context

• Target file: src/cache/OptimisticCache.ts
• Background revalidation should not run more than once concurrently per cache key