refactor: board-owned display init, zero cfg in application code

- Each board now exports DisplayDriver type and create_display() async fn
- ESP32-S3 boards share SSD1306 create_display in board/esp32s3.rs
- RAK WisBlock has its own SSD1306 create_display (different I2C type)
- Wio Tracker has SH1106 create_display (different display IC)
- Removed all cfg_if from display.rs — calls board::create_display()
- Removed display init retry logic (single attempt, return None on failure)
- ZERO board-specific #[cfg(feature)] outside board/, host/mod.rs, and hal/

src/board/heltec_v4.rs

@@ -5,7 +5,10 @@ use super::traits::LoRaBoard;
 use crate::hal::esp32s3 as mcu;
 
 #[allow(unused_imports)] // Re-exported for other modules (display, radio, usb tasks)
-pub use super::esp32s3::{DisplayI2c, DisplayParts, RadioDriver, RadioParts, UsbDriver, UsbParts};
+pub use super::esp32s3::{
+    create_display, DisplayDriver, DisplayI2c, DisplayParts, RadioDriver, RadioParts, UsbDriver,
+    UsbParts,
+};
 
 // ── Board implementation ────────────────────────────────────────────