ci(notebooks): diff Colab oracle against committed snapshots

Extend notebook_validator.py with a colab-diff subcommand that
fetches three files from googlecolab/backend-info:

  pip-freeze.gpu.txt   -> snapshot at scripts/data/colab_pip_freeze.gpu.txt
  apt-list-gpu.txt     -> snapshot at scripts/data/colab_apt_list.gpu.txt
  os-info-gpu.txt      -> snapshot at scripts/data/colab_os_info.gpu.txt

Each file is parsed with a format-specific parser (pip ==, apt
listing, free-form os-info) and compared against the committed
snapshot. The diff reports NEW / REMOVED / CHANGED keys per file.

Wired into Notebooks CI two ways:
- PR-time static job: advisory step (continue-on-error: true) so
  upstream Colab rotations surface in the PR check UI without
  blocking authors.
- Daily static-with-pypi cron: --strict step so backend-info drift
  fails the cron within ~24h and the maintainer can refresh the
  snapshots intentionally.

Catches the same bug classes the existing R-INST-002/003/004/005
rules catch, but earlier: when Colab bumps libcudnn / Python /
torch wheels, we hear about it before a notebook breaks.

Add baseline snapshots from current backend-info HEAD: 1136 apt
packages, 4 os-info entries, 720 pip-freeze entries.

scripts/data/colab_os_info.gpu.txt

@@ -0,0 +1,9 @@
+# Do not modify this file directly; it is generated by extract_colabx_testing_tarballs.sh via
+#  $ (lsb_release -ds;python --version;) > os-info-gpu.txt
+# Be aware that this list does not necessarily reflect the current state of the
+# staging or production container, but rather the state as of the most recent
+# submitted CL where extract_colabx_testing_tarballs.sh was run.
+Ubuntu 22.04.5 LTS
+Python 3.12.13
+R version 4.5.3 (2026-03-11) -- "Reassured Reassurer"
+julia version 1.12.6