Add a file-system backend for the certificate chain cache

## Why

The in-memory cache does not survive a restart, and the CLI starts a fresh process on every command, so nothing is saved across invocations.

## What

Add a `FileCertificateVerifierCache` implementation of `CertificateVerifierCache` in `mithril-client` for non-WASM targets.

## How

- [ ] Store each certificate in a separate file
- [ ] Use a folder for the committed cache
- [ ] Use a folder by certificate chain validation id for staging (support pruning?)
- [ ] Invalidate the cache entry upon parsing error of a certificate file
- [ ] Upon committing, the staged certificates are moved the to the committed cache (atomic files move, do we need some Mutex to commit staging area sequentially?)

Start in the mithril-client implementation of CertificateVerifierCache and review the existing in-memory cache contract, then inspect how the CLI creates and commits cache entries. Define the file layout and staging behavior from the checklist, including parse-error invalidation, pruning, and safe commit semantics; the work is done when each requirement is covered for non-WASM targets.

Written by the indexing model from the issue text.