feat: Add verified committee cache

[itsyaasir]

Summary

• add an async CommitteeCache contract for trusted custom backends
• provide an idempotent in-memory cache that rejects conflicting committees
• let anchored committee resolution use a default or caller-provided cache
• resume from authenticated cached committees and cache new committees only after checkpoint verification
• keep trusted-node resolution separate from the anchored cache

Checkpoint summaries remain temporary verification material and are not retained. Persistent implementations can choose their own internal representation while preserving committee integrity and network separation.

Impact

Repeated anchored resolutions can avoid replaying committee lineage from the trust anchor. Applications may supply database, filesystem, browser, or other cache backends without moving network access into ProofVerifier.

Validation

• cargo test -p poi-rs -- --test-threads=1
• cargo check -p poi-rs
• cargo clippy -p poi-rs --all-targets -- -D warnings
• cargo doc -p poi-rs --no-deps
• cargo fmt -p poi-rs -- --check
• git diff --check

Closes #296