…auge reset (review ai-dynamo#12/ai-dynamo#13/ai-dynamo#19)

Hot-path quality fixes — none change a scaling decision.

ai-dynamo#12 pipeline durations use Clock.monotonic(), not Clock.now()
  Clock's contract reserves now() for wall-clock timestamps and monotonic()
  for duration measurement. Six duration sites (predict latency, fan-out
  call latency, whole-tick duration) used now(); under WallClock a backward
  NTP step mid-tick distorted the latency/duration histograms. Switched all
  six to monotonic(). VirtualClock.monotonic() is synced to trace time in
  replay, so replay/test behavior is unchanged.

ai-dynamo#13 ProposeResult derives result_kind + enforces the oneof
  ProposeResult carries the same accept/override/reject oneof as the stage
  responses but, unlike them, had no model_post_init — so building it the
  natural way (override=...) left result_kind='' and the proto round-trip
  came back 'override', breaking round-trip equality; a two-payload oneof
  violation also went unchecked. Extracted the derive+validate logic into a
  shared _derive_result_kind() helper used by both _StageOneofResponse and
  ProposeResult. +round-trip test (derive + oneof-violation reject).

ai-dynamo#19 override_active gauge reset covers errored plugins
  _emit_override_active reset the gauge only for plugins in plugin_results;
  a plugin whose call raised is absent from that list, so a 1 it set on a
  prior tick lingered. Now reset every ATTEMPTED plugin id (triggered +
  inherited) before setting the contributors.

828 planner tests pass (+1 round-trip test).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>