A self-contained `src/library/pools.ts`: a generic `BoundedPool` concurrency
limiter and a `RequestPools` bundle of three at the design's caps (metadata 10,
content 5, thumbnails 25, each overridable). Within a pool, on-demand work is
served before background/precache work, first-come-first-served within a
priority; a task submitted under a `key` already pending is shared rather than
run twice, and the key frees the moment that task settles. The slot is held for
the task's whole lifetime, so a request that retries internally stays counted
against the cap between attempts.
Pools only; nothing wires them into `Library` yet — that is a later unit.
Model: opus-4-8
sneak