fix(chain): skip coinbase txs with a `last_seen` in canonicalization

fix(chain): skip coinbase txs with a `last_seen` in canonicalization

A coinbase tx cannot be in the mempool, so a `last_seen` value on one is
bad data and must not make it canonical. Previously this was only a
`debug_assert!`, so release builds canonicalized it as unconfirmed.

A coinbase that is an ancestor of a canonical tx, or is assumed
canonical, stays canonical even if no anchor is held for it. As that is
a reachable state, `CanonicalTxOut::is_mature` no longer asserts on it
and returns `false` as maturity cannot be proven.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>