Add updatable ADAU1701 program via flash partition and HTTP API.

Decouple the SigmaStudio RAM download from compiled-in adau1701_program.c:
DRAD v1 blobs in the dsp partition with embedded fallback, POST /api/dsp/program,
and host-tested blob parse/serialize helpers.

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

Software/Firmware/ADAU1701-Firmware/README.md

@@ -15,8 +15,13 @@ SigmaStudio project **DigiRadio**, IC1 = ADAU1701. Shipped with DigiRadio firmwa
 
 ## Runtime (fw 0.8.3)
 
-- **Boot:** ESP32 replays `default_download_IC_1()` over I2C via
-  `adau1701::Adau1701Driver` on every power-up.
+- **Boot:** ESP32 replays the active `DspProgram` over I2C via
+  `adau1701::Adau1701Driver` on every power-up. Source order: valid blob in
+  the `dsp` flash partition, else the embedded export from this folder
+  (`EmbeddedDspProgramSource`).
+- **Program update:** `POST /api/dsp/program` with a `DRAD` v1 blob (see
+  `Software/tools/pack_dsp_program.py`). Device reboots and loads the new
+  script on next boot.
 - **Live control:** six-band PEQ, input mixer, master volume, stereo/bass
   enhancement overlays — safeload via `audio::AudioService` and REST
   `/api/audio/*`; profile persisted in encrypted NVS.
@@ -37,6 +42,13 @@ Parameter map: `components/drivers/adau1701/include/adau1701/Adau1701ParamMap.hp
 Do **not** use SigmaStudio *Link Compile Download* on DigiRadio — export only;
 the ESP32 programs the DSP at every power-up.
 
+## DRAD v1 blob (flash / HTTP upload)
+
+Layout: magic `DRAD`, version u16 (=1), write count u16, CRC32 (IEEE over
+writes section), then for each write: address u16, length u16, data bytes.
+Pack from JSON with `Software/tools/pack_dsp_program.py` or use
+`core::serializeDspProgramBlob()` in host tests.
+
 ## Documentation
 
 - Design: `docs/manual/ch-sigmastudio.tex`

Software/Firmware/ADAU1701-Firmware/SigmaStudioFW.h

@@ -36,19 +36,14 @@ void sigma_studio_set_device(void* i2cDevHandle);
  *
  * @param    devAddress  SigmaStudio device address (0x68 write addr).
  * @param    address     16-bit target address in DSP memory map.
- * @param    length      Payload length in bytes.
+ * @param    length      Payload length in bytes (may exceed 255).
  * @param    pData       Payload bytes.
  */
 void SIGMA_WRITE_REGISTER_BLOCK(unsigned char devAddress,
                                 unsigned int address,
-                                unsigned char length,
+                                unsigned int length,
                                 ADI_REG_TYPE* pData);
 
-/**
- * @brief    Run the DigiRadio SigmaStudio default download sequence.
- */
-void adau1701_run_default_download(void);
-
 /** Safeload data register base (0x0810..0x0814). */
 #define ADAU1701_SAFELOAD_DATA_BASE 0x0810U
 /** Safeload address register base (0x0815..0x0819). */