Managing curves cache

[louispt1]

The curves are handled on hard memory in a /tmp folder in the repository as a caching strategy. At the moment there is no size/eviction policy so the /tmp folder could accumulate large numbers of curve files.

Potential solutions:

• Enable simple file existence caching (uncomment & harden the early return), plus optional checksum freshness check.
• Add an in-memory LRU layer for recently accessed curves (configurable max entries/MB).
• Introduce a manifest (JSON) per scenario with metadata: timestamp, source ETag/version, length, checksum.
• Periodic cleanup utility: remove stale files beyond age / scenarios no longer referenced.