Stop stack mapping after one genome pass with a start value

[alcides]

Summary

• Change stack genotype→phenotype mapping to stop only when both hold:
  1. a start-symbol value has been produced, and
  2. the genome has been consumed at least once (len(dna) codon reads).
• Track codon reads on ListWrapper (consumed / completed_one_pass()).
• Remember the last good start-symbol value so later constructor pops cannot wipe the phenotype before return.
• failures_limit only applies before the first start-symbol value; afterwards mapping continues until one full genome pass.

Test plan

• pytest tests/nxt/stack/stack_stop_condition_test.py tests/nxt/stack/stack_test.py
• pytest tests/nxt/umad_test.py
• CI green