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>

common/params.cc

@@ -250,7 +250,14 @@ std::unordered_map<std::string, uint32_t> keys = {
     
     {"CarSpeedLimitWarning", PERSISTENT},
     {"CarSpeedLimitLiteral", PERSISTENT},
-    
+    {"CarIsMetric", PERSISTENT},
+
+    {"ClearpilotSpeedDisplay", PERSISTENT},
+    {"ClearpilotSpeedLimitDisplay", PERSISTENT},
+    {"ClearpilotHasSpeed", PERSISTENT},
+    {"ClearpilotIsMetric", PERSISTENT},
+    {"ClearpilotSpeedUnit", PERSISTENT},
+
     // {"SpeedLimitLatDesired", PERSISTENT},
     // {"SpeedLimitVTSC", PERSISTENT},