[Hackathon] hiten: add typed FreshnessProof model

[Hiten0305l]

Summary

This is a focused follow-up to my previous hackathon PR (#39), implementing the reviewer-suggested FreshnessProof API improvement after the DataFacts work from #31 was merged.
This PR promotes FreshnessProof from a plugin-local implementation in cid_facts.py to a first-class shared type in nest_core.types and updates the merged cid_facts implementation to use the shared model.

What

• Add FreshnessProof as a shared model in nest_core.types.
• Update the merged cid_facts implementation to use the shared model instead of maintaining a plugin-local definition.
• Update the existing tests to import and validate the shared model from nest_core.types.

Why

Previously, FreshnessProof was defined only inside cid_facts.py, making it an implementation detail of a single plugin.
Promoting it to nest_core.types makes it:

• a first-class public API type,
• statically typed and IDE-discoverable,
• reusable across plugins, validators, scenarios, and future extensions, and
• simpler for verifier implementations to consume consistently.

This follows the reviewer suggestion to promote FreshnessProof into a shared typed model while keeping the change small and focused.

Scope

This PR is intentionally limited to the FreshnessProof API improvement.

Verification

• ✅ make ci-local

uv sync
uv run ruff check .
uv run ruff format --check .
uv run pyright
uv run pytest -v