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/car/hyundai/carstate.py

@@ -212,6 +212,7 @@ class CarState(CarStateBase):
 
     # self.params_memory.put_int("CarSpeedLimitLiteral", self.calculate_speed_limit(cp, cp_cam))
     self.params_memory.put_float("CarSpeedLimit", self.calculate_speed_limit(cp, cp_cam) * speed_conv)
+    self.params_memory.put("CarIsMetric", "1" if self.is_metric else "0")
     self.params_memory.put_float("CarCruiseDisplayActual", cp_cruise.vl["SCC11"]["VSetDis"])
     
 
@@ -420,6 +421,7 @@ class CarState(CarStateBase):
     # print(self.calculate_speed_limit(cp, cp_cam))
     # self.params_memory.put_float("CarSpeedLimitLiteral", self.calculate_speed_limit(cp, cp_cam))
     self.params_memory.put_float("CarSpeedLimit", self.calculate_speed_limit(cp, cp_cam) * speed_factor)
+    self.params_memory.put("CarIsMetric", "1" if self.is_metric else "0")
 
     # CLEARPILOT: telemetry logging — disabled, re-enable when needed
     # speed_limit_bus = cp if self.CP.flags & HyundaiFlags.CANFD_HDA2 else cp_cam