Fix Si4684 pitch distortion: uncalibrated crystal (CTUN/XTAL_FREQ)

Root cause of FM/DAB "stonata" audio: xtalCtun=31/nominal XTAL_FREQ
defaults were never measured against this board's actual crystal
(Abracon ABM8-19.200MHZ-10-1-U-T, CL=10pF, two external 15pF load
caps). Fixed via CTUN trim by ear (31->0) plus a new live calibration
loop that reads the Si4684's own FM_RSQ FREQOFF field (broadcast
carriers are GPS-locked, so it's a free precision frequency reference)
to trim XTAL_FREQ with no lab equipment. Converged to CTUN=0,
XTAL_FREQ=19199750 Hz, residual -3/-4 ppm cross-checked on two
stations.

New tools/infrastructure (kept, not one-off):
- Si4684Driver::recalibrateXtal() + POST /api/tuner/xtal-calibrate:
  live crystal re-trim without an ESP32 reflash.
- FM_RSQ FREQOFF exposed as "freqoff_ppm" in GET /api/tuner/status.
- tools/si4684_xtal_calibration.py: automates the trim loop.
- tools/si4684_antenna_calibration.py: AN851 Appendix A ANTCAP/VARM/VARB
  sweep tool (same session, separate calibration).
- CONFIG_ESP32_I2S_TEST_TONE (off by default): isolates Si4684-specific
  audio issues from shared-downstream ones by writing a tone directly
  over the I2S bus the Si4684 also uses.
- SIGMA_WRITE_REGISTER_BLOCK/sigma_safeload_block now retry on NACK and
  verify via read-back instead of firing I2C writes blind.
- FM de-emphasis set to European 50us (was left at the US 75us default).
- DAB_ACF_ENABLE restored to its previous 0x0000 with a citation
  explaining why (tested the datasheet default of 0x0003, made things
  audibly worse).

See docs/si4684-rf-investigation-report.md's 2026-08-23 entry for the
full elimination chain and docs/TODO.md for follow-up work (persisting
calibration results to EEPROM instead of requiring a firmware edit).

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

Software/main/Kconfig.projbuild

@@ -17,4 +17,17 @@ config I2S_SDATA_PROBE
         input trace (or an equivalent test point) -- enabling this without the
         rewire just reads whatever GPIO16 happens to be floating/connected to.
 
+config ESP32_I2S_TEST_TONE
+    bool "ESP32-generated sine tone over the shared I2S TX channel"
+    default n
+    help
+        Continuously writes a sine wave to esp32_i2s_sink (the same shared
+        BCLK/LRCLK the Si4684 uses on its own SDATA_IN0 pin, but over
+        SDATA_IN1). Diagnostic: if this sounds clean while FM/DAB is
+        distorted, the ADAU1701's I2S input receiving is fine in general and
+        the problem is specific to the Si4684 side. Remember the ESP32
+        mixer channel is muted (-96 dB) by default in the stored audio
+        profile -- raise it via PUT /api/audio/profile to actually hear
+        this.
+
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