[PoI]: Add golden fixtures and compatibility tests

[itsyaasir]

Feature description

Parent epic: #289

PoI milestone: Milestone 5 - Compatibility and test coverage

Add golden proof fixtures and compatibility tests for Proof of Inclusion.

The tests should protect proof format version 1 as a durable verification contract and make intentional proof-format changes explicit.

Motivation

PoI proofs must remain verifiable after SDK, API, and implementation details evolve. Golden fixtures and negative verification tests are the guardrail that keeps proof format version 1 stable.

Requirements

1. Add golden transaction, object, and event proof fixtures.
2. Verify golden fixtures offline.
3. Test that unsupported proof versions return a clear error.
4. Test anchored committee walking from the genesis blob.
5. Test that the cache stores only verified committee material.
6. Test that trusted-node mode does not write trusted-node committees into the anchored cache.
7. Test gRPC construction for transaction, object, and event proofs from a test cluster.
8. Test rejection of mismatched checkpoint contents, transaction effects, events, object references, and event sequences.
9. Treat intentional version 1 fixture changes as compatibility decisions, not routine snapshot updates.

Open questions

• Should fixtures live inside the Rust crate, a shared test-data directory, or both?

Are you planning to do it yourself in a pull request?

Yes