docs: aggiornati ADR 002 e 003 per SPI e audio routing

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+# ADR-003: I2S Audio Routing Architecture
+
+- Status: Accepted
+- Date: 2026-08-01
+- Decision Type: Audio Architecture
+
+## Context
+
+HubAudio integrates multiple digital audio sources and destinations:
+
+Sources:
+
+- ESP32-S3 streaming subsystem
+- Si4684 radio receiver
+- Bluetooth receiver
+- Optical digital input
+
+Destinations:
+
+- Analog audio output through CODEC/DAC
+- Bluetooth transmission
+- Optical digital output
+
+A deterministic and scalable digital audio routing architecture is required.
+
+## Decision
+
+The ADAU1467 is the central routing element of the Audio Domain.
+
+Each major audio device is assigned to a dedicated serial audio interface.
+
+The preferred architecture is:
+
+
+One audio device = One dedicated I2S interface
+
+
+The ADAU1467 operates as the audio timing reference.
+
+External audio devices operate as I2S slaves whenever supported.
+
+## Audio Input Allocation
+
+| ADAU1467 Port | Device | Function |
+|---|---|---|
+| SDATA_IN0 | ESP32-S3 | Network audio stream |
+| SDATA_IN1 | Si4684 | Radio audio |
+| SDATA_IN2 | DECODEC | Optical digital input |
+| SDATA_IN3 | Bluetooth RX | Wireless audio input |
+
+
+ESP32-S3 --------
+Si4684 ---------
+DECODEC ----------+---- ADAU1467
+BT RX ---------/
+
+
+## Audio Output Allocation
+
+| ADAU1467 Port | Device | Function |
+|---|---|---|
+| SDATA_OUT0 | CODEC/DAC | Analog audio output |
+| SDATA_OUT1 | Bluetooth TX | Wireless audio output |
+| SDATA_OUT2 | ENCODEC | Optical digital output |
+| SDATA_OUT3 | Reserved | Future expansion |
+
+             ADAU1467
+
+                 |
+    +------------+------------+
+    |            |            |
+  CODEC        BTTX       ENCODEC
+
+## Clocking
+
+The ADAU1467 provides the master audio timing reference.
+
+The audio clock domain consists of:
+
+- MCLK
+- BCLK
+- LRCLK
+
+All connected audio peripherals must operate synchronized to this clock
+domain.
+
+## Consequences
+
+### Positive
+
+- Deterministic audio timing
+- Simple routing model
+- Independent audio channels
+- Easier debugging
+- Future expansion capability
+
+### Negative
+
+- Higher pin usage
+- More PCB routing resources required
+- Peripheral selection must consider I2S slave capability
+
+## Rationale
+
+HubAudio prioritizes signal integrity, maintainability and deterministic
+audio behavior over maximum bus utilization.
+
+The ADAU1467 acts as the central audio router of the system.