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/mod.rs

@@ -0,0 +1,16 @@
+//! Host communication: unified USB/UART transport.
+//!
+//! Exports a single `host_task` regardless of transport.
+//! The cfg dispatch is contained entirely within this module.
+
+pub mod framing;
+
+cfg_if::cfg_if! {
+    if #[cfg(feature = "heltec_v3")] {
+        mod uart;
+        pub use uart::host_task;
+    } else {
+        mod usb;
+        pub use usb::host_task;
+    }
+}