Add Slice 4 tuner stack with AGENTS-compliant core types and HTTP API.

Introduces Si4684/ADAU1701 drivers, TunerService, FrequencyKHz,
SeekDirection, /api/tuner routes without file-scope globals, host tests,
and firmware blob tooling (binaries remain gitignored).

Co-authored-by: Cursor <cursoragent@cursor.com>

Software/Firmware/README.md

@@ -0,0 +1,20 @@
+# DigiRadio — companion-chip firmware assets
+
+Binary and SigmaStudio exports loaded by the ESP32 at every boot.
+
+| Directory | Chip | Contents |
+|-----------|------|----------|
+| `ADAU1701-Firmware/` | ADAU1701 SigmaDSP | SigmaStudio export (`DigiRadio_IC_1.h`, …) — RAM program |
+| `Si4684-Firmware/` | Si4684 DAB+/FM tuner | ROM patch + DAB + FM application images (`.bin`) |
+
+## Si4684 blobs
+
+`rom_patch_016.bin` and `dab_firmware.bin` are extracted from the
+[PE5PVB SI4684-DAB-Receiver](https://github.com/PE5PVB/SI4684-DAB-Receiver)
+project. `fm_firmware.bin` comes from the Skyworks Si4684 eval firmware pack
+(see `tools/fetch_si4684_firmware.py --si46xx-dir`).
+
+## ADAU1701 export
+
+Generated by Analog Devices SigmaStudio for the DigiRadio DSP schematic.
+Loaded on every boot (no self-boot EEPROM on this board).