Drive the ESP32-C5 station over USB or BLE, send CAM or VAM, signed or not

The app now speaks the station-link protocol of the new obu-firmware.
Esp32Link picks the transport from Settings (UsbSerialTransport or the new
BleLinkTransport), tells the previous firmware from the new one by its
heartbeat, and runs the session: STATION_CONFIGURE with the current pseudonym
MAC (which also starts the board's radio), CREDENTIALS_PROVISION of the
bundled demo chain when the board has no ticket, then per message a
POTI_UPDATE and a BTP_DATA_REQUEST. Received messages still arrive as
V2X_RX frames, so the receive side is unchanged. A board on the previous
firmware keeps working for CAM over USB.

Settings > Connection > ESP32-C5: link USB-C or Bluetooth, transmit CAM or
VAM, "Sign outgoing messages" (on by default). The connection card, top bar
and dashboard show the link in use, the pairing passkey and signing counters.

- VAM: VamUperCodec (TS 103 300-3 V2.3.1, bytes checked against asn1tools)
  and VamGenerationRules (clause 6.4, Tables 16/17).
- BLE: the firmware's GATT layout (service 0000C175-...), MTU 517, pairing
  and encryption settled before any other operation (short timeouts during
  pairing made it loop), backoff between attempts, reasons on the card.
- Clock: a PoTi goes to the board once per new fix and never moves the
  board's clock backwards except for a real correction (>= 60 s); stale and
  wobbling fix times made the board answer time_regression and restart its
  stack every few seconds. GnssTimeSource keeps the last measured phone-clock
  error while GNSS time drops out indoors: the bench phone is 14 minutes fast,
  and falling back to it made every transmitted timestamp jump by that much.
- assets/demo-chain.vcr: throwaway, not EU-registered demo chain generated
  2026-09-23 (AT B80B49387A4C12EB, psid 36 and 638). Its private key ships
  with the app on purpose; receivers verifying against the EU trust list
  drop what it signs.
- Bluetooth permissions requested at start-up on Android 12+.

StationLinkTest pins the codec to bytes from the colleague's Python
implementation (microbu_link/messages.py). 103 unit tests pass.

app/src/test/java/com/hawhamburg/micr0bu/VamUperCodecTest.kt

@@ -0,0 +1,59 @@
+package com.hawhamburg.micr0bu
+
+import com.hawhamburg.micr0bu.domain.asn1.VamContent
+import com.hawhamburg.micr0bu.domain.asn1.VamUperCodec
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+/**
+ * Pins the VAM encoder against bytes this code did not produce.
+ *
+ * ## Where the expected bytes come from
+ * asn1tools 0.167, compiling the ETSI modules vanetza-idf ships (asn1/release2:
+ * TS102894-2v241-CDD.asn, TS103300-3v231/VAM-PDU-Descriptions.asn and its motorcyclist container),
+ * encoding the same values as a Python dict: the same toolchain the colleague's reference VBS
+ * (microbu-esp32c5/station-link/python/microbu_link/vbs.py) builds its VAMs with. Station
+ * 0x12345678, fix at Unix ms 1790170000123 (generationDeltaTime 45819), 53.5546667 N
+ * 10.0223889 E, 5.43 m/s, heading 123.4 deg, -1.26 m/s^2; every confidence and the altitude
+ * unavailable, as the encoder sends them.
+ */
+class VamUperCodecTest {
+
+    private fun ByteArray.hex(): String = joinToString("") { "%02x".format(it) }
+
+    private fun content(includeLowFrequency: Boolean, accuracyM: Float? = 3.0f) = VamContent(
+        stationId = 0x12345678,
+        timestamp = 1_790_170_000_123L,
+        latitude = 53.5546667,
+        longitude = 10.0223889,
+        accuracyM = accuracyM,
+        speedMps = 5.43,
+        headingDeg = 123.4,
+        accelerationMps2 = -1.26,
+        includeLowFrequency = includeLowFrequency,
+    )
+
+    @Test
+    fun `VAM with the low-frequency container matches asn1tools`() {
+        assertEquals(
+            "031012345678b2fb400aac85a15b8a18ec88f30f3708eddd0f8002697e087ff24f322220",
+            VamUperCodec.encode(content(includeLowFrequency = true)).hex(),
+        )
+    }
+
+    @Test
+    fun `VAM without the low-frequency container matches asn1tools`() {
+        assertEquals(
+            "031012345678b2fb000aac85a15b8a18ec88f30f3708eddd0f8002697e087ff24f30",
+            VamUperCodec.encode(content(includeLowFrequency = false)).hex(),
+        )
+    }
+
+    @Test
+    fun `unknown accuracy encodes the confidence ellipse as unavailable`() {
+        assertEquals(
+            "031012345678b2fb000aac85a15b8a18ec8fffffff08eddd0f8002697e087ff24f30",
+            VamUperCodec.encode(content(includeLowFrequency = false, accuracyM = null)).hex(),
+        )
+    }
+}