Si4684 RF investigation report; fix ADAU1701 I2C reliability under long safeload bursts

- Add docs/si4684-rf-investigation-report.md: full record of the FM/DAB no-lock
  investigation (crystal, front-end matching, ANTCAP sweep, continuity, leading
  EP solder-defect hypothesis, PCBWay report sent) plus the separate audio
  profile NVS bug found and partially fixed this session.
- Fix sigma_i2c_write() (SigmaStudioFW.c): no retry on I2C failure meant a single
  transient NACK anywhere in a long safeload burst (EQ apply = ~55 sequential
  transactions) aborted the whole sequence. Added a 3-attempt retry.
- Add granular failure logging (AudioService::applyProfileToDsp/persistProfile,
  Adau1701Driver::applyMixer/applyEq, NvsAudioProfileStore::saveProfile error
  codes) to isolate the remaining NVS-side audio profile save failure.
- Si4684Driver::tuneFm gains an optional ANTCAP argument (default 0 = unchanged
  auto-tune behavior) used during this session's front-end matching sweep.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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

@@ -203,6 +203,11 @@ public:
      *
      * @dname    tuneFm
      * @param    frequency  FM centre frequency in kHz.
+     * @param    antCap     FM_TUNE_FREQ ARG4/5 ANTCAP (AN649 Command 0x30).
+     *                      0 = automatic (varactor set from FE_VARM/VARB
+     *                      properties); 1-128 bypasses auto-tune and forces
+     *                      the on-chip antenna varactor directly (AN851
+     *                      Appendix A calibration procedure).
      * @return   Ok on success, or Si4684Error::WrongBand / TuneFailed.
      * @pubstate sends FM_TUNE_FREQ and waits for STC (AN649).
      *
@@ -210,7 +215,7 @@ public:
      * @date     2026-07-06
      */
     [[nodiscard]] std::expected<void, Si4684Error> tuneFm(
-        core::FrequencyKHz frequency);
+        core::FrequencyKHz frequency, std::uint8_t antCap = 0U);
 
     /**
      * @brief    seekFm — seek FM in the given direction.