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/esp-synopsys-usb-otg/src/transition.rs

@@ -0,0 +1,41 @@
+use usb_device::endpoint::{EndpointAddress, EndpointType};
+
+/// USB endpoint descriptor information.
+pub struct EndpointDescriptor {
+    /// Endpoint address.
+    pub address: EndpointAddress,
+
+    /// Endpoint transfer type.
+    pub ep_type: EndpointType,
+
+    /// Maximum packet size.
+    pub max_packet_size: u16,
+
+    /// Poll interval for interrupt endpoints.
+    pub interval: u8,
+}
+
+/// Configuration for an endpoint allocation.
+pub struct EndpointConfig {
+    /// The transfer type of the endpoint to be allocated.
+    pub ep_type: EndpointType,
+
+    /// Maximum packet size for the endpoint to be allocated.
+    pub max_packet_size: u16,
+
+    /// Poll interval for interrupt endpoints.
+    pub interval: u8,
+
+    /// Requests a specific endpoint number. Allocation shall fail if the number is not available.
+    pub number: Option<u8>,
+
+    /// Specifies that the endpoint is the "pair" of another endpoint.
+    ///
+    /// If `ep` is an endpoint in the opposite direction, this means that the endpoint to be
+    /// allocated uses the same transfer type as `ep` but in the opposite direction in all alternate
+    /// settings for the interface.
+    ///
+    /// If `ep` is an endpoint in the same direction, this means that the two endpoints belong to
+    /// different alternate settings for the interface and may never be enabled at the same time.
+    pub pair_of: Option<EndpointAddress>,
+}