Add FM ANTCAP antenna calibration: HTTP sweep control + EEPROM persistence

Confirmed live tonight that the Si4684's automatic front-end tuning
(FE_VARM/VARB properties) is measurably suboptimal on this board: a
129-value ANTCAP sweep (0-128, AN851 Appendix A) at four different FM
frequencies found antcap=102 beats auto-tune by +6 to +11 dB RSSI and
+3 to +11 dB SNR everywhere tested, consistent with the front-end
network component mismatch already logged in
docs/si4684-rf-investigation-report.md — the board's actual matching
network differs from the AN851 reference network those auto-tune
constants were derived from, so a fixed empirical override
compensates for a gap the chip's own algorithm can't see.

Wiring, bottom to top:
- Si4684Driver::tuneFm() already took an antCap byte; threaded it
  through core::ITuner::tuneFm() and si4684::Si4684Tuner::tuneFm() as
  a new parameter (default 0 = auto, unchanged behaviour for every
  existing caller).
- TunerService::tuneFm() takes an optional override instead: omitted,
  it falls back to a new defaultFmAntCap_ member so every ordinary FM
  tune (seek, scan, station recall, live UI) benefits automatically
  once calibrated, not just calls that pass antcap explicitly.
- POST /api/tuner/tune gained an optional "antcap" field for sweeping
  live without touching the saved calibration.
- POST /api/tuner/calibrate-antenna commits a sweep result: writes it
  to the 24AA025E48 EEPROM's user-writable region (word address 0x00,
  separate from the factory-locked EUI-48 at 0xFA-0xFF) via a new
  Eeprom24aa::writeFmAntCap()/readFmAntCap() pair, and immediately
  updates the live TunerService default — no reboot needed to take
  effect, though HardwareBootstrap::boot() also loads it at every
  boot so it survives power cycles. Same net::AntennaCalibration
  function-pointer bridge pattern as PhoneStreamSink/BleProvisioning,
  so components/net stays free of eeprom24aa headers.

Verified end to end on hardware: swept and found 102, saved it via
the new endpoint, confirmed the live default changed immediately,
then power-cycled and confirmed the boot log reports "FM ANTCAP
calibration loaded: 102" and a subsequent default tune reflects it.

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

Software/main/hardware_bootstrap.cpp

@@ -80,7 +80,21 @@ bt1035::Bt1035Driver gBt1035(
     });
 
 core::DeviceIdentity gDeviceIdentity = core::DeviceIdentity::unknown();
+std::optional<std::uint8_t> gFmAntCapCalibration;
 bool gReady = false;
+
+/**
+ * @brief    makeEeprom — construct a transient EEPROM handle onto the
+ *           shared I2C bus, matching the one built inline in boot().
+ */
+[[nodiscard]] eeprom24aa::Eeprom24aa makeEeprom()
+{
+    auto* busHandle =
+        static_cast<i2c_master_bus_handle_t>(gAdau1701.i2cBusHandle());
+    return eeprom24aa::Eeprom24aa(
+        busHandle,
+        static_cast<std::uint8_t>(board::pins::Eeprom24aaAddr));
+}
 } // namespace
 
 std::expected<void, HardwareBootError> HardwareBootstrap::boot()
@@ -102,11 +116,7 @@ std::expected<void, HardwareBootError> HardwareBootstrap::boot()
         }
     }
 
-    auto* busHandle =
-        static_cast<i2c_master_bus_handle_t>(gAdau1701.i2cBusHandle());
-    eeprom24aa::Eeprom24aa eeprom(busHandle,
-                                  static_cast<std::uint8_t>(
-                                      board::pins::Eeprom24aaAddr));
+    eeprom24aa::Eeprom24aa eeprom = makeEeprom();
     if (auto identity = eeprom.readDeviceIdentity(); identity) {
         gDeviceIdentity = std::move(*identity);
         ESP_LOGI(kTag, "unit serial %.*s",
@@ -117,6 +127,19 @@ std::expected<void, HardwareBootError> HardwareBootstrap::boot()
         ESP_LOGW(kTag, "EUI-48 read failed — using fallback identity");
     }
 
+    if (auto antCap = eeprom.readFmAntCap(); antCap) {
+        gFmAntCapCalibration = *antCap;
+        if (gFmAntCapCalibration) {
+            ESP_LOGI(kTag, "FM ANTCAP calibration loaded: %u",
+                     static_cast<unsigned>(*gFmAntCapCalibration));
+        } else {
+            ESP_LOGI(kTag, "FM ANTCAP not calibrated — using chip auto-tune");
+        }
+    } else {
+        ESP_LOGW(kTag, "FM ANTCAP calibration read failed — using chip "
+                       "auto-tune");
+    }
+
     if (auto audioResult = gAudioService.loadAndApply(); !audioResult) {
         ESP_LOGW(kTag, "ADAU1701 profile apply failed");
     }
@@ -175,4 +198,22 @@ const core::DeviceIdentity& HardwareBootstrap::deviceIdentity() noexcept
     return gDeviceIdentity;
 }
 
+std::optional<std::uint8_t> HardwareBootstrap::fmAntCapCalibration() noexcept
+{
+    return gFmAntCapCalibration;
+}
+
+bool HardwareBootstrap::saveFmAntCapCalibration(std::uint8_t antCap)
+{
+    eeprom24aa::Eeprom24aa eeprom = makeEeprom();
+    if (auto written = eeprom.writeFmAntCap(antCap); !written) {
+        ESP_LOGW(kTag, "FM ANTCAP calibration write failed");
+        return false;
+    }
+    gFmAntCapCalibration = antCap;
+    ESP_LOGI(kTag, "FM ANTCAP calibration saved: %u",
+             static_cast<unsigned>(antCap));
+    return true;
+}
+
 } // namespace hardware