fix(chain): correct ChainPosition ordering for wallet transaction display

fix(chain): correct ChainPosition ordering for wallet transaction display

Previously, unconfirmed transactions never seen in mempool would appear
before those with mempool timestamps due to derived Ord implementation.

Changes:
- Manual Ord impl: confirmed < unconfirmed < never-in-mempool
- At same height: transitive confirmations < direct (potentially earlier)
- Simplify FullTxOut ordering to only essential fields
- Add comprehensive tests and documentation
- Update CanonicalTx and FullTxOut to use manual Ord with A: Ord bound