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/components/core/src/TunerJson.cpp

@@ -129,6 +129,10 @@ std::string serializeTunerStatusJson(const TunerStatus& status)
         if (status.fmSnrDb) {
             out << ",\"snr_db\":" << static_cast<int>(*status.fmSnrDb);
         }
+        if (status.fmFreqOffBppm) {
+            out << ",\"freqoff_ppm\":"
+                << (static_cast<int>(*status.fmFreqOffBppm) * 2);
+        }
         if (status.fmStereo) {
             out << ",\"stereo\":" << (*status.fmStereo ? "true" : "false");
         }
@@ -364,4 +368,41 @@ parseAntennaCalibrationJson(std::string_view json)
     return req;
 }
 
+std::expected<XtalCalibrationRequest, ParseError>
+parseXtalCalibrationJson(std::string_view json)
+{
+    if (json.find('{') == std::string_view::npos) {
+        return std::unexpected(ParseError::InvalidJson);
+    }
+    // Defaults match Si4684Driver::boot()'s own defaults -- a request that
+    // only wants to change one parameter can omit the others.
+    XtalCalibrationRequest req = {};
+    req.ibias = 72U;
+    req.ctun = 31U;
+    req.xtalFreqHz = 19200000U;
+
+    unsigned long ibias = 0U;
+    if (extractJsonUint(json, "ibias", ibias)) {
+        if (ibias > 127U) {
+            return std::unexpected(ParseError::InvalidJson);
+        }
+        req.ibias = static_cast<std::uint8_t>(ibias);
+    }
+
+    unsigned long ctun = 0U;
+    if (extractJsonUint(json, "ctun", ctun)) {
+        if (ctun > 63U) {
+            return std::unexpected(ParseError::InvalidJson);
+        }
+        req.ctun = static_cast<std::uint8_t>(ctun);
+    }
+
+    unsigned long xtalFreqHz = 0U;
+    if (extractJsonUint(json, "xtal_freq_hz", xtalFreqHz)) {
+        req.xtalFreqHz = static_cast<std::uint32_t>(xtalFreqHz);
+    }
+
+    return req;
+}
+
 } // namespace core