fix(notebook): make BinderDiffuser import robust in Colab

The previous install cell guarded the clone+install with 'if [ ! -d ]',
which meant a partially-failed first run would leave the repo cloned but
the package un-installed; subsequent runs hit ModuleNotFoundError.

Fixes:
  - Always run pip install -e (idempotent, fast on re-run)
  - git pull on existing clone so notebook updates land
  - Inject /content/BinderDiffuser/src onto sys.path as a safety net for
    cases where editable installs don't propagate to a running kernel
  - Add a verification cell that imports binderdiffuser before any
    later cell uses it; fails fast with a clear message
  - Replace removed Bio.PDB.Polypeptide.three_to_one (gone in Biopython
    1.80+) with a local AA3->AA1 map; handles non-canonical residues
    (MSE/SEC/PYL) and falls back to X
  - Guard ranked-design printout against None ipTM / pAE_interface
  - Drop redundant 'import shutil' from the download cell

Author: deepmind11