Pin AlphaFold structures for StructAlign test to avoid version drift (#48)

tmalign.txt was generated from AlphaFold v4 models; the EBI API now
serves v6, whose coordinates differ enough to fail the alignment
quality assertions. Add pinned v4 PDB files for K7N701 and K7N778
under test/PDBs/ and load them directly in the StructAlign test,
keeping the download functionality test unaffected.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: timholy