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.hpp

@@ -17,7 +17,9 @@
 
 #include "bt1035/Bt1035Driver.hpp"
 
+#include <cstdint>
 #include <expected>
+#include <optional>
 
 namespace audio {
 class AudioService;
@@ -137,6 +139,35 @@ public:
      * @date     2026-07-07
      */
     [[nodiscard]] static const core::DeviceIdentity& deviceIdentity() noexcept;
+
+    /**
+     * @brief    fmAntCapCalibration — saved FM antenna calibration, if any.
+     *
+     * @dname    fmAntCapCalibration
+     * @return   Calibrated ANTCAP (0-128) read from the 24AA025E48 during
+     *           boot(), or nullopt if never calibrated / the read failed.
+     * @pubstate reads gFmAntCapCalibration; set once during boot().
+     *
+     * @author   Michele Bigi
+     * @date     2026-08-19
+     */
+    [[nodiscard]] static std::optional<std::uint8_t>
+    fmAntCapCalibration() noexcept;
+
+    /**
+     * @brief    saveFmAntCapCalibration — persist a new FM ANTCAP to EEPROM.
+     *
+     * @dname    saveFmAntCapCalibration
+     * @param    antCap  Value found via a calibration sweep (0-128).
+     * @return   true on success, false on an I2C failure.
+     * @pubstate writes the 24AA025E48 user region and gFmAntCapCalibration.
+     *           Does not itself change any live tuner state — callers must
+     *           also call TunerService::setDefaultFmAntCap() to apply it.
+     *
+     * @author   Michele Bigi
+     * @date     2026-08-19
+     */
+    [[nodiscard]] static bool saveFmAntCapCalibration(std::uint8_t antCap);
 };
 
 } // namespace hardware