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>

selfdrive/manager/manager.py

@@ -89,6 +89,7 @@ def manager_init(frogpilot_functions) -> None:
   params_memory.put("VpnEnabled", "1")
   params_memory.put("DashcamFrames", "0")
   params_memory.put("DashcamState", "stopped")
+  params_memory.put("LatRequested", "0")
   params_memory.put("ModelFps", "20")
   params_memory.put("ModelStandby", "0")
   params_memory.put("ShutdownTouchReset", "0")