factor into workspace, improve features & deps

examples/README.md

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+# usbd-uac2 Examples
+
+This repository contains example implementations of a USB Audio Class 2 (UAC2) device.
+
+Two example backends are provided, both based on the LPCXpresso55S28 demo board, using the onboard WM8904 DAC:
+
+- **Interrupt-driven example** (`lpc55s28-evk`)
+- **DMA-based example** (`lpc55s28-evk-dma`)
+
+
+## Examples Overview
+
+### Interrupt-driven (`lpc55s28-evk`)
+
+Running at 32bit/48khz. It can't keep up at 96khz. Works on USBFS and USBHS.
+
+This is a minimal implementation intended to demonstrate the fundamental
+structure of the class driver. It fills a `bbqueue` as data comes in from USB,
+and drains it into the I2S FIFO in the I2S interrupt. This requires a lot of
+time-critical CPU work managing buffers. Particularly, the USB peripheral driver
+uses a lot of interrupt-free critical sections which can cause late interrupts
+and underruns.
+
+This is intended primarily as a learning/reference implementation
+
+### DMA-based (`lpc55s28-evk-dma`)
+
+Running at 32bit/96khz. Works on USBFS and USBHS.
+
+A more realistic and robust implementation using DMA. It fills a static ring
+buffer as data comes in from USB, while the DMA chases it around the ring,
+draining into the TX FIFO. This is efficient and decouples interrupt latency
+from data delivery. It uses the DMA interrupt to track consumed slots, but as
+long as the USB doesn't catch up to the read slot before this happens, there is
+a lot of slack for other things to be happening.
+
+This is a more useful demonstration, but is still lacking correct handling of
+edge cases, error conditions and so on you would want in a fully fleshed out
+implementation. Particularly, it behaves poorly in underrun, since the DMA will
+keep emitting from the ring regardless of whether the data is valid, which
+sounds terrible.
+
+## Running the Examples
+
+You can flash and run either example using `cargo embed`:
+
+```sh
+cargo embed --release --example lpc55s28-evk --features usbfs