test(chain): cover taint propagation and eligibility classification

test(chain): cover taint propagation and eligibility classification

- taint propagates through a pending output's unsettled ancestry
- is_settled alone decides the settled boundary, even for unconfirmed outputs
- taint never crosses a settled ancestor
- an immature coinbase is classified apart from a settled output
- two UTXOs sharing a tainting ancestor are both untrusted (shared taint cache)
- classify_outpoints skips spent and out-of-view outpoints

Co-authored-by: 志宇 <hello@evanlinjin.me>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>