v0.4.1

Added

• SingGuardClient(thinking="slow") and thinking="cascade": the argus-cull
  singguard service's modes. slow reasons with no fast first-line verdict,
  so the reasoning is not anchored to a verdict made before it; cascade runs
  fast on every image and slow only on the ones fast flags, and slow's
  verdict stands. THINKING_MODES lists them. A SingGuard server without a
  mode answers HTTP 400, raised as GuardCheckFailed per check.
• GuardResult.thinking: the mode the server says it judged in (None from a
  server that does not say). The argus-cull service can be forced server-side
  to a mode other than the one asked for, and this records which it used.