Keep Exacto codon validation behind the lazy Varcode import boundary #439

The initial implementation in PR #438 imported varcode.effects.codon_tables.STANDARD at module load time. Since the public Exacto reader is exported by topiary, a plain import topiary now eagerly imports Varcode.

The full local gate caught this with the existing test_import_topiary_does_not_import_varcode; the run was stopped after 2,848 passing tests and that failure. This is an unreleased regression in #438, not a problem in a published Topiary version.

Load the codon table only when validating translated sequence. Keep the existing lazy-import test and verify native table/fragment workflows as well. This preserves the lightweight import boundary and avoids loading the genomics stack for table-only users.