Fix remaining Si4684 ARG-offset bugs: seek, DAB services, RSQ/RDS/DIGRAD acks

writeCommand() had no way to send a non-zero ARG1, since it always
hardcoded ARG1=0x00 before the caller's payload. Every command whose real
ARG1 needed to carry a flag (STCACK, INTACK, SERTYPE, DIGRAD/EVENT ack) or
whose payload needed to start at ARG1 instead of ARG2 was silently broken:

- seekFm(): SEEKUP/WRAP never reached the chip (always ARG2=0x00), so
  hardware seek always searched down/no-wrap; masked by the existing
  100 kHz software-step fallback in Si4684Tuner.
- startDabService()/stopDabService(): SERVICE_ID/COMPONENT_ID shifted one
  byte right of their real ARG4-11 positions, with SERTYPE landing where
  the spec requires a fixed 0x00.
- readDabServiceData(): same shift, plus STATUS_ONLY was bit3 (0x08)
  instead of the correct bit4 (0x10).
- clearFmStc(), readFmRsq(), readFmRds(), fetchDabServiceList(),
  readDabDigRadStatus(), readDabEventStatus(): these AN649 commands have
  only ARG1 and no ARG2 at all, so the old two-argument writeCommand()
  could never carry their ack/status flags — clearFmStc()'s STCACK never
  fired in this driver's history (masked by FM_TUNE_FREQ/FM_SEEK_START
  auto-clearing STC per their own spec).

writeCommand() gains a fourth parameter, arg1 (default 0x00, preserving
every already-correct call site); each caller above now passes its flag
through arg1 instead of the payload array.

Confirmed live: first locked:true and first genuine hardware seek (not
software-fallback) in this driver's history — 87.5 -> 98.3 MHz, RSSI +12
dBuV, SNR +14 dB.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0178rASQ6ZETPMUamvpoR2KR

Software/components/drivers/si4684/include/si4684/Si4684Driver.hpp

@@ -408,7 +408,8 @@ private:
     [[nodiscard]] std::expected<void, Si4684Error> readRaw(
         std::span<std::uint8_t> buffer);
     [[nodiscard]] std::expected<void, Si4684Error> writeCommand(
-        Command cmd, const std::uint8_t* payload, std::size_t length);
+        Command cmd, const std::uint8_t* payload, std::size_t length,
+        std::uint8_t arg1 = 0x00U);
     [[nodiscard]] std::expected<void, Si4684Error> hostLoadBlob(
         const core::IFirmwareBlobReader& blob, std::size_t chunkPayload);
     [[nodiscard]] std::expected<void, Si4684Error> configureAfterBoot(