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/manager/manager.py

@@ -89,6 +89,12 @@ def manager_init(frogpilot_functions) -> None:
   params_memory.put("VpnEnabled", "1")
   params_memory.put("ModelStandby", "0")
   params_memory.put("ModelStandbyTs", "0")
+  params_memory.put("CarIsMetric", "0")
+  params_memory.put("ClearpilotSpeedDisplay", "")
+  params_memory.put("ClearpilotSpeedLimitDisplay", "0")
+  params_memory.put("ClearpilotHasSpeed", "0")
+  params_memory.put("ClearpilotIsMetric", "0")
+  params_memory.put("ClearpilotSpeedUnit", "mph")
   params.clear_all(ParamKeyType.CLEAR_ON_ONROAD_TRANSITION)
   params.clear_all(ParamKeyType.CLEAR_ON_OFFROAD_TRANSITION)
   if is_release_branch():