feat(metrics): observe felis_start_duration_seconds across the start lifecycle

Wire the fourth mandated §23 metric to a real producer. The histogram
spans two reconcile passes, so anchor and observation must persist in
status:

- Add status.startRequestedAt, set once on the first Starting reconcile
  of a start attempt and cleared on Stopped so the next start re-anchors.
- Observe felis_start_duration_seconds exactly when readiness is first
  reached (ReadySignalAt - StartRequestedAt), guarded so a server that
  reaches ready without a Starting pass records nothing.
- Mirror the field into the deepcopy and the structural CRD schema so the
  apiserver does not prune it on patchStatus round-trips.
- Promote prometheus/client_golang and client_model to direct deps now
  that the operator and its tests import them.

Tests drive a step clock through Starting -> Running asserting the exact
observed duration, and through Running -> Stopped asserting the metric is
observed once and the anchor clears.

deploy/crd/felis.lolicon.best_minecraftservers.yaml

@@ -383,6 +383,16 @@ spec:
                 description: ReadySignalAt is when the first RCON probe succeeded.
                 format: date-time
                 type: string
+              startRequestedAt:
+                description: |-
+                  StartRequestedAt is when the current start attempt was first observed
+                  (the first Starting reconcile after desiredState=Running). It anchors the
+                  felis_start_duration_seconds histogram (spec §23): the operator observes
+                  ReadySignalAt-StartRequestedAt the moment readiness is first reached, then
+                  clears this on stop so the next start re-anchors. Persisted in status
+                  because the two endpoints fall in different reconcile passes.
+                format: date-time
+                type: string
             type: object
         type: object
     served: true