[Feature]: Offline pre-fetch & stale-while-revalidate cartridge caching

[hyperpolymath]

Problem / motivation

On-demand cartridge fetching adds cold-start latency and is a single point of failure if boj-server-cartridges is unreachable/rate-limited.

Proposed solution

• Bundle a pre-warmed cache of the top-20 cartridges in the release/container.
• Stale-while-revalidate: run the cached cartridge immediately while revalidating in the background.

Rough scope

Medium. Runtime hardening.

Part of #274. Re-file of #263 (mis-templated).