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]>

testdata/gen_fixtures.py

@@ -0,0 +1,20 @@
+#!/usr/bin/env python3
+"""Regenerate the Go test vectors in crypto_test.go from the reference client.
+
+    pip install --target=/tmp/pybydenv pybyd==0.0.75 cryptography
+    PYTHONPATH=/tmp/pybydenv python3 gen_fixtures.py
+
+If the Go tests fail after a pybyd upgrade, run this: BYD rotating the bangcle
+tables is the likely cause, and bangcle_tables.bin needs replacing too
+(pybyd/data/bangcle_tables.bin inside the wheel).
+"""
+from pybyd._crypto.bangcle import BangcleCodec
+from pybyd._crypto.hashing import md5_hex, pwd_login_key, sha1_mixed, compute_checkcode
+
+c = BangcleCodec()
+for p in ["", "hello", '{"functionType":"pwdLogin"}', "ünïcodé ✓"]:
+    print(f"{p!r} -> {c.encode_envelope(p)}")
+print("md5Hex(hunter2)      ", md5_hex("hunter2"))
+print("pwdLoginKey(hunter2) ", pwd_login_key("hunter2"))
+print("sha1Mixed(abc)       ", sha1_mixed("abc"))
+print("checkcode            ", compute_checkcode({"zeta": "1", "alpha": "2"}))