[PoI] : Persist authenticated committee history in the PoI CLI

[itsyaasir]

Problem

The poi verify command currently constructs committee resolution with a fresh in-memory cache. Every invocation therefore repeats the authenticated committee walk from genesis to the proof epoch, and any committees authenticated before a transient failure are discarded when the process exits.

Proposed scope

• Provide a file-backed CommitteeCache adapter that can be reused by the CLI and the advanced committee-cache example.
• Store committee entries under the IOTA configuration directory, for example poi/committee-cache/<chain-identifier>/epoch-<epoch>.bcs.
• Preserve the complete CommitteeCacheKey, including the trusted genesis ChainIdentifier and epoch.
• Write authenticated committees atomically and never overwrite a conflicting entry.
• Configure poi verify with CommitteeResolution::from_genesis_with_cache for both managed-network and explicit-genesis verification.
• Document the cache location and that deleting it is safe but makes the next verification slower.

Acceptance criteria

• A later CLI invocation reuses committees authenticated by an earlier invocation.
• Mainnet, testnet, devnet, and custom genesis files cannot collide in one backend.
• A failed or interrupted walk preserves committees stored before the failure.
• Malformed or conflicting cache entries fail closed with a useful error.
• No new CLI flag is required for the default behavior.

Out of scope

• Concurrent epoch-summary prefetching for the first cold walk.
• Network retry or backoff policy.
• Genesis download pinning and validation tracked by H2.