Use the global Filecoin proof parameter cache

[Kubuxu]

Problem

foc-devnet stores Filecoin proof parameters in its private data directory:

~/.foc-devnet/docker/volumes/cache/filecoin-proof-parameters

The path comes from foc_devnet_proof_parameters() in src/paths.rs and is used by the downloader, startup prerequisite checks, status command, and the Lotus/lotus-miner/Curio container bind mounts.

Proof parameters are not specific to a foc-devnet run or to foc-devnet itself. Keeping a private copy prevents reuse of an existing system-wide Filecoin parameter cache and can cause another large download and duplicate disk usage when other Filecoin tools are installed.

Expected behavior

Use the standard global Filecoin proof parameter cache on the host (normally /var/tmp/filecoin-proof-parameters) and bind-mount that directory into the containers at the existing /var/tmp/filecoin-proof-parameters path.

Acceptance criteria

• An existing global Filecoin proof parameter cache is reused without downloading another foc-devnet-specific copy.
• Parameter downloading, prerequisite validation, status reporting, and all relevant container bind mounts resolve to the same global host directory.
• The chosen path works on every supported host platform, with permissions and any migration from the current foc-devnet-private cache handled or documented.