DGM20 control for Linux: protocol library, CLI and local web UI

Reverse-engineered from Maono Link 4.0.88 (DGM20_OLD.dll) and verified
against a DGM20 (352f:0105, firmware 2.1.2).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

PROTOCOL.md

@@ -0,0 +1,74 @@
+# DGM20 control protocol
+
+These notes come from Maono Link 4.0.88 for Windows. The app loads a per-model plugin, and the
+plugin for USB `352f:0105` is `DGM20_OLD.dll`. Every value below was checked against a real mic
+(firmware 2.1.2) unless it is marked otherwise.
+
+## Transport
+
+The mic has USB audio interfaces 0-2 and a HID interface 3. Interface 3 carries a vendor usage page
+(`0xFF01`) with four reports:
+
+| report | direction | size |
+|--------|-----------|------|
+| 0x50   | out       | 15   |
+| 0x51   | out       | 63   |
+| 0x10   | in        | 15   |
+| 0x11   | in        | 63   |
+
+The app uses only 0x51 (requests) and 0x11 (replies). On Linux the interface is a hidraw node: a
+65-byte `write()` (the report id plus 64 data bytes) sends a request, and `read()` returns replies.
+
+## Packet
+
+```
+offset  size  field
+0       1     report id: 0x51 request, 0x11 reply
+1       1     0xC4
+2       1     length of bytes 1..end, including the checksum
+3       2     0x0000
+5       1     type
+6       n     payload, u16 little-endian words
+6+n     2     checksum, u16 LE = -(sum of bytes 1 .. 5+n)
+```
+
+The rest of the 64 bytes is zero-padded.
+
+| type | request payload | reply |
+|------|-----------------|-------|
+| 2    | (reply only)    | write ack: `status` (0 = ok) |
+| 3    | `addr, value` pairs | type-2 ack. The mic also sends type-3 packets on its own when a setting changes on the hardware, such as the touch pad. |
+| 4    | `addr`          | `addr, value` |
+| 5    | `addr, count, data...` | bulk write. The app uses it only to write the serial number; don't. |
+| 6    | `addr, count`   | `addr, count, values...` (range read) |
+
+## Registers
+
+| reg    | meaning | values |
+|--------|---------|--------|
+| 0x0003 | model name, 8 words, ASCII | `"DGM20"` |
+| 0x000B | firmware version, 3 words | `2, 1, 2` |
+| 0x001E | serial number, up to 20 words | |
+| 0x1008 | DFU mode (read) | 0 = normal |
+| 0x100A | factory reset | write 1 |
+| 0x2022 | mute | 0 / 1 |
+| 0x2026 | headphone monitoring | 2000 = on, 3000 = off |
+| 0x2027 | noise cancellation | 0 / 1 |
+| 0x2028 | noise cancellation strength | 0-100 |
+| 0x202A | RGB ring brightness | 0-100 (the app does not show it for this model) |
+| 0x202C | vocal enhancement | 0 / 1 |
+| 0x204B | level-meter enable | not answered by firmware 2.1.2 |
+
+Maono Link reads settings once, in this order: 0x204B, then 0x2022, then a range read of 0x2027
+(count 2), then 0x2026, then 0x202C. After that it relies on type-3 packets the mic sends on its
+own.
+
+## Present in the plugin but not used for this model
+
+The DLL shares widgets with the PD200X plugin, so it contains EQ and RGB-colour code. That code
+is never instantiated for the DGM20, and the mic does not answer reads of these registers. They
+are listed here for anyone exploring.
+
+- **EQ:** 5 parametric bands, each 5 words, starting at 0x204D + 5·i. The HPF is at 0x2066 and the
+  LPF at 0x206B. Each band is `[enable, type, freq Hz, gain dB×10+2000, Q×100]`.
+- **Light on/off:** 0x2036. **Colour index:** 0x2038.