LayeredCache

A two-tier cache composing a fast memory layer over an optional persistent layer.

Mirrors SwiftyNetwork's LayeredCache: reads consult memory first, fall back to the persistent layer, and promote a persistent hit back into memory — preserving the original timestamp when the memory layer is a TimestampedCache. Writes go through to both layers.

changes reflects writes and removals against this layered view; read-time promotions are not reported, since the logical value did not change. A write can also evict other keys from the backing caches, so observers must re-read their keys on every notification from this view.

Operations through this instance are serialized across both tiers, so a slow disk read cannot promote stale data over a newer write or resurrect a removed entry. Access the backing caches through this instance to retain that guarantee; independently mutating them bypasses its lock.

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