content: add 4 Austria canons (banking/food-safety/policy/disaster), re-verify 3 aging canons

[dbwls99706]

Gap filled

Austria (at) had only 5 country-scoped canons, all in
communication/emergency/legal/medical/visa, and zero in banking,
food-safety, policy, or disaster - domains where deadends.dev has a
defensible moat over generic LLM answers. This PR adds one canon in
each of those four gap domains.

Canons added

• banking/blocked-account-not-mandatory/at - Austria has no legal
  requirement to open a German-style Sperrkonto (blocked account) for the
  Aufenthaltsbewilligung - Studierende student residence permit; salary
  slips, pension/insurance proof, investment capital, bank statements, or a
  Haftungserklärung are all accepted alternatives.
• food-safety/raw-milk-vending-machine-must-boil/at - Austria's
  Rohmilchverordnung (BGBl. II Nr. 106/2006, §5) legally requires raw milk
  sold via farm vending machines to carry a "boil before consumption"
  warning; it is not equivalent to pasteurized supermarket milk.
• policy/deposit-return-any-retailer/at - Since 1 Jan 2025, Austria's
  Einwegpfand deposit-bottle system requires any retailer selling that
  packaging type to accept returns (no receipt, not limited to the
  purchase branch), while standalone vending machines and postal couriers
  are excluded from the take-back duty.
• disaster/avalanche-level-3-most-fatal/at - Danger level 3
  ("considerable"/Erheblich) on the European avalanche scale sounds like a
  moderate middle rating but accounts for 54% of Austrian avalanche
  accidents over a 10-year average per ÖKAS - far more than any other
  level.

Primary sources used

• migration.gv.at, oesterreich.gv.at, oead.at (banking)
• ris.bka.gv.at (Rohmilchverordnung full text), ages.at,
  lebensmittelaufsicht-oberoesterreich.org (food-safety)
• oesterreich.gv.at, wien.gv.at, wko.at (policy)
• oesterreich.gv.at, alpinesicherheit.at (ÖKAS), avalanches.org
  (EAWS) (disaster)

Each canon cites 2-3 of the sources above across its dead_ends and
workarounds; no Reddit/blog content was used as a sole source.

Re-verified aging canons (bucket seed at)

Per python -m generator.reverify --seed at, after git fetch origin --prune confirmed a clean (non-INCOMPLETE) claim scan:

• cuda/nvcc-not-found/cuda12-linux - re-checked all 3 source URLs
  (NVIDIA docs), still live and accurate. Date bumped, no content change.
• pip/yanked-version-warning/pip24-linux - pip.pypa.io/en/stable/topics/yanked/
  now 404s; the yanked-release documentation moved to
  docs.pypi.org/project-management/yanking/, which still confirms the
  claimed behavior (yanked releases are skipped by installers unless
  pinned with ==/===). Source URL updated in both dead_ends entries
  and the workarounds entry that cited it; date bumped.
• python/zerodivisionerror/py311-linux - re-checked all 3
  docs.python.org source URLs, still live and accurate. Date bumped, no
  content change.

Validation

All run locally and passing:

• ruff check generator/ tests/ - clean
• python -m pytest tests/ -q - 369 passed
• python -m generator.validate --data-only - PASSED
• python -m generator.build_site - 2505 error pages generated
• python -m generator.validate --site-only - PASSED

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