v3.46.1 — `cs doctor` shows when you're behind; a failed refresh keeps the per-model limits

cs doctor shows when you're behind; a failed refresh keeps the per-model limits.

• cs doctor adds an auto-upgrade line (on, off, or blocked by another
  install owning cs / a Windows pip or pipx install) and a latest line read
  from the updater's cache: an available version, up to date, or unknown when
  the last check is over 7 days old. No network call (#69, thanks @bpavlina).
• A single timeout or keychain hiccup no longer blanks the per-model limits
  segment. The last good limits stay for up to 10 minutes after the last
  successful fetch; retry backoff is unchanged (#70, thanks @bpavlina).
• The macOS installer refuses to fall back to the unsigned tar archive when a
  3.46+ release is missing its notarized disk image.
• Troubleshooting docs explain how uv installs stuck on 3.35.2–3.42.x get
  unstuck (#71).