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/adau1701/src/Adau1701Driver.cpp

@@ -241,12 +241,14 @@ namespace adau1701
                                          mixer.si4684Right);
             !result)
         {
+            ESP_LOGW(kTag, "applyMixer: si4684 input safeload failed");
             return result;
         }
         if (auto result = setInputVolume(core::MixSource::Esp32, mixer.esp32Left,
                                          mixer.esp32Right);
             !result)
         {
+            ESP_LOGW(kTag, "applyMixer: esp32 input safeload failed");
             return result;
         }
         if (auto result =
@@ -254,10 +256,17 @@ namespace adau1701
                              mixer.mixLeft);
             !result)
         {
+            ESP_LOGW(kTag, "applyMixer: st0 safeload failed");
             return result;
         }
-        return safeloadGain(static_cast<unsigned>(ADDR_STMIXER1_ST1_VOLUME),
-                            mixer.mixRight);
+        if (auto result = safeloadGain(
+                static_cast<unsigned>(ADDR_STMIXER1_ST1_VOLUME), mixer.mixRight);
+            !result)
+        {
+            ESP_LOGW(kTag, "applyMixer: st1 safeload failed");
+            return result;
+        }
+        return {};
     }
 
     std::expected<void, Adau1701Error> Adau1701Driver::setEqBand(
@@ -344,6 +353,8 @@ namespace adau1701
             if (auto result = setEqBand(*index, band.gain, band.center, band.q);
                 !result)
             {
+                ESP_LOGW(kTag, "applyEq: band %u safeload failed",
+                        static_cast<unsigned>(i));
                 return result;
             }
         }

Software/components/drivers/adau1701/src/SigmaStudioFW.c

@@ -16,6 +16,7 @@
 #include "driver/i2c_master.h"
 #include "freertos/FreeRTOS.h"
 #include "freertos/semphr.h"
+#include "freertos/task.h"
 
 #include <string.h>
 
@@ -135,9 +136,22 @@ static int sigma_i2c_write(unsigned int reg, const unsigned char* data,
     buf[0] = (unsigned char)((reg >> 8) & 0xFFU);
     buf[1] = (unsigned char)(reg & 0xFFU);
     memcpy(buf + 2U, data, length);
-    const esp_err_t err =
-        i2c_master_transmit(s_dev, buf, (size_t)(2U + length), 1000);
-    return err == ESP_OK ? 0 : -1;
+
+    // Long safeload sequences (e.g. one EQ band = 5 params x 2 writes +
+    // trigger) chain dozens of back-to-back transactions; a single
+    // transient NACK anywhere in that chain used to abort the whole
+    // sequence. Retry a few times before giving up -- cheap and matches
+    // how every other bus driver in this codebase tolerates bus noise.
+    static const int kMaxAttempts = 3;
+    esp_err_t err = ESP_FAIL;
+    for (int attempt = 0; attempt < kMaxAttempts; ++attempt) {
+        err = i2c_master_transmit(s_dev, buf, (size_t)(2U + length), 1000);
+        if (err == ESP_OK) {
+            return 0;
+        }
+        vTaskDelay(pdMS_TO_TICKS(2));
+    }
+    return -1;
 }
 
 /*