feat: add random meters in demo

dsp408-protocol.md

@@ -617,26 +617,81 @@ delay = delay_min + (raw / 255) × (delay_max - delay_min)
 
 | Field | Value |
 |---|---|
-| **Purpose** | Per-channel level + peak metering |
-| **Size** | 3 bytes per channel |
-| **Total** | 12 channels × 3 bytes = 36 bytes |
+| **Purpose** | Per-channel RMS + peak metering, with clip flags |
+| **Payload size** | 38 bytes (exactly; any other length is an error) |
+| **Layout** | 12 channels × 3 bytes = 36 bytes, followed by 2 clip-flag bytes |
+| **Channel order** | Channels 0–3 are inputs, channels 4–11 are outputs, each mapped in enum order |
+
+#### Channel record (3 bytes, at offset `i = channel × 3`)
 
 | Byte offset | Field | Format |
 |---|---|---|
-| 0–1 | Level | IEEE754 float16, little-endian |
-| 2 | Peak | 1-byte peak value (format not specified in source) |
+| `i + 0` | RMS low byte | Low byte of the RMS float16 |
+| `i + 1` | Shared high byte | High byte of **both** the RMS and Peak float16s |
+| `i + 2` | Peak low byte | Low byte of the Peak float16 |
+
+Both values are IEEE 754 half-precision floats (little-endian pairs). The peak is not a separate 1-byte value: it reuses byte `i + 1` as its high byte, so its byte order is reversed relative to the offset order.
 
-**Encoding (value → raw):**
 ```text
-level_bytes = float16_encode(level)   // 2 bytes, little-endian
-peak_byte   = peak_encode(peak)       // format undefined — see note below
-raw = level_bytes + peak_byte
+rms_raw  = float16_decode(lo = payload[i],     hi = payload[i + 1])
+peak_raw = float16_decode(lo = payload[i + 2], hi = payload[i + 1])
 ```
 
-**Decoding (raw → value):**
+If a float16 fails to decode, the raw value defaults to `0.0`, which converts to `MIN_DB` (-60 dB).
+
+#### Clip flags (bytes 36–37)
+
+| Channel index | Flag location |
+|---|---|
+| 0–3 | `payload[36]`, bit `index` |
+| 4–11 | `payload[37]`, bit `index - 4` |
+
 ```text
-level = float16_decode(raw[0:2])      // little-endian
-peak  = peak_decode(raw[2])           // format undefined — see note below
+clip = (flags_byte & (1 << bit)) != 0
+```
+
+#### Raw → dB conversion
+
+Raw values are converted with a piecewise-linear curve, then clamped.
+
+| Constant | Value | Meaning |
+|---|---|---|
+| `X0` | -25.0 dB | Breakpoint |
+| `Y0` | 1.0 | Raw value at the breakpoint |
+| `K1` | 0.0104 | Raw units per dB below the breakpoint (`raw < Y0`) |
+| `K2` | 0.0208 | Raw units per dB above the breakpoint (`raw >= Y0`) |
+| `MIN_DB` | -60.0 dB | Lower clamp |
+| `MAX_DB` | 12.0 dB | Upper clamp |
+
+**Decoding (raw → dB):**
+```text
+if raw < Y0:  db = X0 + (raw - Y0) / K1
+else:         db = X0 + (raw - Y0) / K2
+db = clamp(db, MIN_DB, MAX_DB)
+```
+
+**Encoding (dB → raw):** the inverse of the above. Values outside the clamp range are not recoverable.
+```text
+db = clamp(db, MIN_DB, MAX_DB)
+if db < X0:   raw = Y0 + (db - X0) * K1
+else:         raw = Y0 + (db - X0) * K2
+```
+
+#### Full decode
+
+```text
+require len(payload) == 38
+
+for index in 0..11:
+    i        = index * 3
+    rms_db   = raw_to_db(float16_decode(payload[i],     payload[i + 1]))
+    peak_db  = raw_to_db(float16_decode(payload[i + 2], payload[i + 1]))
+    clip     = index < 4 ? payload[36] bit index
+                         : payload[37] bit (index - 4)
+    meter[index] = { rms: rms_db, peak: peak_db, clip }
+
+inputs  = meter[0..3]   → InputChannel enum order
+outputs = meter[4..11]  → OutputChannel enum order
 ```
 
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