feat: speed limit sign UI and speed processing pipeline

New speed_logic.py module converts raw CAN speed limit and GPS speed
into display-ready params. Called from controlsd (live) and
bench_onroad (bench) at ~2Hz. UI reads params to render:
- Current speed (top center, hidden when 0 or no GPS)
- MUTCD speed limit sign (lower-left, normal + nightrider styles)
- Unit-aware display (mph/kph based on CAN DISTANCE_UNIT)

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

selfdrive/ui/qt/onroad.h

@@ -51,6 +51,7 @@ public:
 private:
   void drawText(QPainter &p, int x, int y, const QString &text, int alpha = 255);
   void drawSpeedWidget(QPainter &p, int x, int y, const QString &title, const QString &speedLimitStr, QColor colorSpeed, int width = 176);
+  void drawSpeedLimitSign(QPainter &p);
 
   QVBoxLayout *main_layout;
   QPixmap dm_img;
@@ -59,6 +60,14 @@ private:
   bool nightriderMode = false;
   int displayMode = 0;
   QString speedUnit;
+
+  // ClearPilot speed state (from params_memory, updated ~2Hz)
+  bool clpHasSpeed = false;
+  QString clpSpeedDisplay;
+  QString clpSpeedLimitDisplay;
+  QString clpSpeedUnit;
+  int clpParamFrame = 0;
+
   float setSpeed;
   float speedLimit;
   bool is_cruise_set = false;