refactor: unified host/ communication module, cfg-free run()

- Created host/ module with internal cfg dispatch (USB vs UART)
- Moved usb/task.rs → host/usb.rs, uart/task.rs → host/uart.rs
- Moved protocol_io.rs → host/framing.rs
- Renamed usb_task/uart_task to host_task (uniform name)
- Removed all host-comm cfg from main.rs
- The run() function now has ZERO board-specific conditional compilation
- Deleted old usb/, uart/, protocol_io.rs modules

src/host/framing.rs

@@ -0,0 +1,124 @@
+//! Shared COBS protocol helpers for USB and UART transports.
+//!
+//! Both `usb_task` and `uart_task` use these helpers for frame
+//! accumulation, response encoding, and command routing.
+
+use defmt::warn;
+
+use crate::channel::{CommandChannel, DisplayCommand, DisplayCommandChannel, ResponseChannel};
+use crate::protocol::{Command, ErrorCode, Response};
+
+/// Maximum COBS frame size (bytes).
+pub const MAX_FRAME: usize = 512;
+
+// Worst case: tag(1) + rssi(2) + snr(2) + len(2) + payload(256) = 263 bytes.
+// COBS adds ceil(263/254)+1 = 3 bytes. 266 < 512. Plenty of room.
+const _: () = assert!(
+    MAX_FRAME >= crate::protocol::MAX_PAYLOAD + 64,
+    "MAX_FRAME too small for max payload + COBS overhead"
+);
+
+/// Decodes COBS-framed commands from a byte stream.
+pub struct CobsDecoder {
+    buf: [u8; MAX_FRAME],
+    len: usize,
+}
+
+impl CobsDecoder {
+    pub const fn new() -> Self {
+        Self {
+            buf: [0u8; MAX_FRAME],
+            len: 0,
+        }
+    }
+
+    /// Reset the accumulator (discard partial frame).
+    pub fn reset(&mut self) {
+        self.len = 0;
+    }
+
+    /// Feed a chunk of bytes, calling `on_command` for each decoded command.
+    pub fn feed(&mut self, data: &[u8], mut on_command: impl FnMut(Command)) {
+        for &byte in data {
+            if byte == 0x00 {
+                // End of COBS frame — decode and dispatch
+                if self.len > 0 {
+                    let mut decode_buf = [0u8; MAX_FRAME];
+                    if let Some(decoded_len) =
+                        ucobs::decode(&self.buf[..self.len], &mut decode_buf)
+                    {
+                        if let Some(cmd) = Command::from_bytes(&decode_buf[..decoded_len]) {
+                            on_command(cmd);
+                        }
+                    }
+                }
+                self.len = 0;
+            } else if self.len < MAX_FRAME {
+                self.buf[self.len] = byte;
+                self.len += 1;
+            } else {
+                // Frame too large — discard
+                self.len = 0;
+            }
+        }
+    }
+}
+
+/// COBS-encode a response into `encode_buf` with trailing 0x00 sentinel.
+///
+/// Returns the slice to send (encoded frame + sentinel), or `None` on overflow.
+pub fn cobs_encode_response<'a>(
+    response: Response,
+    write_buf: &mut [u8; MAX_FRAME],
+    encode_buf: &'a mut [u8; MAX_FRAME],
+) -> Option<&'a [u8]> {
+    let raw_len = response.write_to(write_buf);
+    let encoded_len = ucobs::encode(&write_buf[..raw_len], encode_buf).unwrap_or(0);
+    if encoded_len < encode_buf.len() {
+        encode_buf[encoded_len] = 0x00;
+        Some(&encode_buf[..encoded_len + 1])
+    } else {
+        warn!("COBS encode buffer overflow");
+        None
+    }
+}
+
+/// Route a parsed command to the appropriate handler.
+///
+/// Display and MAC commands are handled locally (response sent immediately).
+/// All other commands are forwarded to the radio task (response sent later).
+/// Both paths feed the same `ResponseChannel`, so the one-outstanding-command
+/// rule in PROTOCOL.md is required to keep solicited responses ordered.
+pub async fn route_command(
+    cmd: Command,
+    commands: &CommandChannel,
+    responses: &ResponseChannel,
+    display_commands: &DisplayCommandChannel,
+    has_display: bool,
+    mac: [u8; 6],
+) {
+    match cmd {
+        Command::DisplayOn => {
+            if has_display {
+                display_commands.send(DisplayCommand::On).await;
+                responses.send(Response::Ok).await;
+            } else {
+                responses.send(Response::Error(ErrorCode::NoDisplay)).await;
+            }
+        }
+        Command::DisplayOff => {
+            if has_display {
+                display_commands.send(DisplayCommand::Off).await;
+                responses.send(Response::Ok).await;
+            } else {
+                responses.send(Response::Error(ErrorCode::NoDisplay)).await;
+            }
+        }
+        Command::GetMac => {
+            responses.send(Response::MacAddress(mac)).await;
+        }
+        other => {
+            commands.send(other).await;
+        }
+    }
+}