Fix Si4684 I2S audio: PIN_CONFIG_ENABLE conflict + ADAU1701 BCLK polarity

Two independent bugs silenced the digital audio path from Si4684 into the
ADAU1701 even after the FM/DAB tune fix produced a real RF lock:

1. Si4684Driver::configureAfterBoot() wrote PIN_CONFIG_ENABLE (0x0800) as
   0x0003, enabling both I2SOUTEN and DACOUTEN. AN649: "only I2SOUTEN or
   DACOUTEN can be enabled at a time. If both enabled, only analog audio
   output is enabled" — the chip silently fell back to its unused analog
   DAC output on every boot. Fixed to 0x8002 (I2SOUTEN + INTBOUTEN),
   matching the value hitech95/si468x_dab_receiver's working ALSA codec
   driver uses (SI468X_PROP_I2S_ENABLED); I2SOUTEN alone (0x0002) was not
   sufficient on this hardware. Also fixed AUDIO_OUTPUT_CONFIG (0x0302),
   which was being written with a stray I2S-enable bit that property does
   not have (its only real field is bit0 MONO).

2. Even with the chip correctly outputting I2S, the ADAU1701 received only
   static. Traced with SIGMA_WRITE_REGISTER_BLOCK live overrides of
   SerialInputRegister (0x081F, baked into the compiled SigmaStudio export
   at ILP=0/IBP=0): IBP=1 (input data clocked on the opposite BCLK edge)
   produced real, recognizable music instead of static on a locked, strong
   FM signal — first confirmed end-to-end audio in this project's history.
   ILP=1 made it worse and was reverted; IBP=1 kept as a runtime override
   in Adau1701Driver::boot().

Remaining noise on top of the music is attributed to antenna quality
(RSSI/SNR fluctuated significantly between retunes of the same station on
the current improvised antenna) — not yet confirmed with a proper antenna.

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

Software/components/drivers/adau1701/src/Adau1701Driver.cpp

@@ -158,6 +158,21 @@ namespace adau1701
             return replay;
         }
 
+        // SerialInputRegister (0x081F) override: bit3 IBP=1, matching the
+        // BCLK edge the Si4684's I2S output actually changes data on
+        // (compiled DSP program default is 0x00 = IBP=0, which produced
+        // pure static on a strong locked signal). ILP=1 was also tried
+        // (0x18) and made it worse (pure white noise again) — IBP alone
+        // (0x08) is the correct override, confirmed live: real, recognizable
+        // music instead of static/noise on a locked FM station.
+        {
+            const unsigned char deviceAddr =
+                static_cast<unsigned char>(pins_.i2cAddr7 << 1);
+            ADI_REG_TYPE serialInFix = 0x08U;
+            SIGMA_WRITE_REGISTER_BLOCK(deviceAddr, 0x081FU, 1U,
+                                       &serialInFix);
+        }
+
         booted_ = true;
         ESP_LOGI(kTag, "SigmaStudio program loaded");
         return {};