Add BYD vehicle telemetry exporter

Go port of the read path of pybyd 0.0.75 (the client behind
jkaberg/hass-byd-vehicle): the bangcle white-box AES envelope, the
signing/checkcode layer, login, vehicle list, and realtime readings.

Scraping runs on a cron schedule rather than per Prometheus request
because each reading wakes the car's T-Box and drains the traction
battery; /metrics serves the last result.

Every numeric field of the realtime payload becomes a gauge, so the
exporter does not need a hand-maintained field list that goes stale
when BYD changes the payload.

Tests pin the wire format against pybyd byte-for-byte and drive the
client end to end against a fake cloud that speaks the real protocol.

Co-Authored-By: Claude Opus 5 <[email protected]>

docker-compose.yml

@@ -0,0 +1,20 @@
+# Deployment stack. Copy to the GitOps compose repo as byd-exporter/docker-compose.yml.
+services:
+  byd-exporter:
+    image: 192.168.1.17:30008/thistine/byd-exporter:latest
+    container_name: byd-exporter
+    restart: always
+    environment:
+      BYD_USERNAME: ${BYD_USERNAME:?set in .env}
+      BYD_PASSWORD: ${BYD_PASSWORD:?set in .env}
+      BYD_REGION: ${BYD_REGION:-sg}
+      BYD_COUNTRY_CODE: ${BYD_COUNTRY_CODE:-TH}
+      BYD_TIME_ZONE: ${BYD_TIME_ZONE:-Asia/Bangkok}
+      # Every scrape wakes the car. Hourly is already fairly aggressive.
+      BYD_SCHEDULE: ${BYD_SCHEDULE:-0 * * * *}
+    networks:
+      - proxy
+
+networks:
+  proxy:
+    external: true