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/net/src/SetupWebServer.cpp

@@ -699,6 +699,57 @@ esp_err_t tunerCalibrateAntennaPostHandler(httpd_req_t* req)
     return httpd_resp_send(req, json.c_str(), json.size());
 }
 
+/**
+ * @brief    tunerXtalCalibratePostHandler — POST /api/tuner/xtal-calibrate.
+ *
+ * Diagnostic-only, 2026-08-23: reboots the Si4684 with new IBIAS/CTUN/
+ * XTAL_FREQ (AN649 §Command 0x01 POWER_UP) without an ESP32 restart or
+ * NVS persistence, so a calibration script can iterate crystal parameters
+ * live. Caller must re-tune afterwards -- this only reboots the chip.
+ * See FM_RSQ_STATUS FREQOFF (GET /api/tuner/status, "freqoff_ppm") for the
+ * measurement this is meant to null out.
+ */
+esp_err_t tunerXtalCalibratePostHandler(httpd_req_t* req)
+{
+    auto* ctx = routeContextFrom(req);
+    if (ctx == nullptr || ctx->antennaCalibration == nullptr
+        || ctx->antennaCalibration->recalibrateXtal == nullptr) {
+        httpd_resp_set_status(req, "503 Service Unavailable");
+        return httpd_resp_send(req, nullptr, 0);
+    }
+
+    std::array<char, 128> body{};
+    if (!readRequestBody(req, body)) {
+        httpd_resp_set_status(req, "400 Bad Request");
+        return httpd_resp_send(req, nullptr, 0);
+    }
+
+    const auto parsed =
+        core::parseXtalCalibrationJson(std::string_view(body.data()));
+    if (!parsed) {
+        const std::string json = core::serializeTunerErrorJson("invalid_json");
+        httpd_resp_set_status(req, "400 Bad Request");
+        httpd_resp_set_type(req, "application/json");
+        return httpd_resp_send(req, json.c_str(), json.size());
+    }
+
+    if (!ctx->antennaCalibration->recalibrateXtal(
+            parsed->ibias, parsed->ctun, parsed->xtalFreqHz)) {
+        const std::string json = core::serializeTunerErrorJson("boot_failed");
+        httpd_resp_set_status(req, "500 Internal Server Error");
+        httpd_resp_set_type(req, "application/json");
+        return httpd_resp_send(req, json.c_str(), json.size());
+    }
+
+    const std::string json =
+        std::string("{\"status\":\"recalibrated\",\"ibias\":")
+        + std::to_string(parsed->ibias) + ",\"ctun\":"
+        + std::to_string(parsed->ctun) + ",\"xtal_freq_hz\":"
+        + std::to_string(parsed->xtalFreqHz) + "}";
+    httpd_resp_set_type(req, "application/json");
+    return httpd_resp_send(req, json.c_str(), json.size());
+}
+
 /**
  * @brief    audioProfileGetHandler — serve GET /api/audio/profile JSON.
  *
@@ -2075,6 +2126,14 @@ std::expected<void, NetError> SetupWebServer::start(
     };
     httpd_register_uri_handler(server_, &tunerCalibrateAntennaUri);
 
+    const httpd_uri_t tunerXtalCalibrateUri = {
+        .uri = "/api/tuner/xtal-calibrate",
+        .method = HTTP_POST,
+        .handler = tunerXtalCalibratePostHandler,
+        .user_ctx = routeCtx,
+    };
+    httpd_register_uri_handler(server_, &tunerXtalCalibrateUri);
+
     const httpd_uri_t audioProfileGetUri = {
         .uri = "/api/audio/profile",
         .method = HTTP_GET,