Refactor USTB onto shared utils/chainlink.py (cleanup)

[spalen0]

Context

Cleanup follow-up. Once utils/chainlink.py exists (foundation issue), slim protocols/ustb/main.py to reuse the shared Chainlink reader + staleness / round helpers instead of its inline implementation. USTB is the pattern these utilities were generalized from, so it should be the reference consumer.

Scope

• Replace inline latestRoundData / staleness / monotonicity logic in protocols/ustb/main.py with the shared utils/chainlink.py helpers.
• Keep behavior identical (same thresholds, same alerts).
• Regression check against current output.

Priority

Low / optional — do after L1 + L2 land.

References

• Foundation issue (utils/chainlink.py).

[spalen0]

Implemented in PR #311: #311

• Refactors USTB Chainlink NAV reads to use utils.chainlink.read_feeds.
• Keeps the existing raw-answer cache key and monotonicity alert behavior.
• Adds focused regression coverage for shared Chainlink feed batching/decoding.