feat: ramp-up delay on lat engagement to prevent commIssue flash

Decouples "tell modeld to go fast" from "steering actually active":
- New LatRequested memory param — controlsd writes when lat would be active
- modeld reads LatRequested (not carControl.latActive) for FPS decision,
  so it switches to 20fps immediately on engage request
- controlsd delays CC.latActive becoming true by 250ms (5 frames @ 20fps)
  after LatRequested goes true, giving downstream services
  (longitudinalPlan, liveCalibration, etc.) time to stabilize at the new rate

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

common/params.cc

@@ -201,6 +201,7 @@ std::unordered_map<std::string, uint32_t> keys = {
     {"TelemetryEnabled", PERSISTENT},
     {"Timezone", PERSISTENT},
     {"TrainingVersion", PERSISTENT},
+    {"LatRequested", PERSISTENT},
     {"ModelFps", PERSISTENT},
     {"ModelStandby", PERSISTENT},
     {"ModelStandbyTs", PERSISTENT},