Use the global Filecoin proof parameter cache

## Problem

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

```text
~/.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.

Start in src/paths.rs at foc_devnet_proof_parameters(), then trace its use by the downloader, startup prerequisite checks, status command, and Lotus, lotus-miner, and Curio container bind mounts. Done means all of these use the standard global host cache, reuse an existing cache without another download, and address supported-platform permissions and migration from the private cache.

Written by the indexing model from the issue text.