feat: add display UI with splash screen, dashboard, and RSSI sparkline

Implement full SSD1306 OLED display support:
- Splash screen on boot (1.5s) with project name, version, board
- Dashboard showing radio state, config, counters, RSSI/SNR
- RSSI history sparkline (64-sample ring buffer, 2px-wide bars)
- DisplayOn/DisplayOff host commands routed via dedicated channel
- Pure render functions against generic DrawTarget (no hardware coupling)
- Display on/off control bypasses radio task entirely

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/lora-phy/src/sx126x/variant.rs

@@ -0,0 +1,47 @@
+use super::DeviceSel;
+
+/// Implement this trait on your custom variant or use provided impls
+pub trait Sx126xVariant {
+    /// whether to use high or low power PA
+    fn get_device_sel(&self) -> DeviceSel;
+
+    /// use dio2 as rf switch output
+    fn use_dio2_as_rfswitch(&self) -> bool {
+        true
+    }
+}
+
+/// Sx1261 uses only LowPowerPA
+pub struct Sx1261;
+impl Sx126xVariant for Sx1261 {
+    fn get_device_sel(&self) -> super::DeviceSel {
+        super::DeviceSel::LowPowerPA
+    }
+}
+
+/// Sx1262 uses only HighPowerPA
+pub struct Sx1262;
+
+impl Sx126xVariant for Sx1262 {
+    fn get_device_sel(&self) -> super::DeviceSel {
+        super::DeviceSel::HighPowerPA
+    }
+}
+
+/// Stm32wl variant.
+pub struct Stm32wl {
+    /// select which output to use. (Switching is not supported)
+    pub use_high_power_pa: bool,
+}
+impl Sx126xVariant for Stm32wl {
+    fn get_device_sel(&self) -> super::DeviceSel {
+        if self.use_high_power_pa {
+            DeviceSel::HighPowerPA
+        } else {
+            DeviceSel::LowPowerPA
+        }
+    }
+    fn use_dio2_as_rfswitch(&self) -> bool {
+        false
+    }
+}