[codex] Prune impossible assembly merge pairs

[iskandr]

Summary

• add an exact k-mer candidate index before greedy assembly pair scoring so obviously impossible sequence pairs are never compared
• keep the existing deterministic merge semantics by leaving VariantSequence.combine as the authoritative merge check
• add a regression test showing disjoint sequences now skip pairwise combine calls entirely

Validation

• ./lint.sh
• ./test.sh

Closes #147

[coveralls]

coverage: 93.391% (-0.009%) from 93.4% — improve-assembly-scaling into master