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/antenna_calibration.cpp

@@ -11,14 +11,28 @@
 #include "antenna_calibration.hpp"
 
 #include "hardware_bootstrap.hpp"
+#include "si4684/Si4684Tuner.hpp"
 
 namespace antenna_calibration {
 
+namespace {
+
+/** Diagnostic-only, 2026-08-23: see net::AntennaCalibration::recalibrateXtal. */
+bool recalibrateXtal(std::uint8_t ibias, std::uint8_t ctun,
+                     std::uint32_t xtalFreqHz)
+{
+    return static_cast<bool>(hardware::HardwareBootstrap::si4684Tuner()
+                                  .recalibrateXtal(ibias, ctun, xtalFreqHz));
+}
+
+} // namespace
+
 net::AntennaCalibration& bridge() noexcept
 {
     static net::AntennaCalibration instance{
         .save = &hardware::HardwareBootstrap::saveFmAntCapCalibration,
         .saveDab = &hardware::HardwareBootstrap::saveDabAntCapCalibration,
+        .recalibrateXtal = &recalibrateXtal,
     };
     return instance;
 }