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/cortex-m-rt/examples/device.rs

@@ -0,0 +1,37 @@
+//! Manually create the interrupts portion of the vector table
+
+#![deny(unsafe_code)]
+#![deny(warnings)]
+#![no_main]
+#![no_std]
+
+extern crate cortex_m_rt as rt;
+extern crate panic_halt;
+
+use rt::entry;
+
+#[entry]
+fn main() -> ! {
+    loop {}
+}
+
+// interrupts portion of the vector table
+#[repr(C)]
+pub union Vector {
+    handler: unsafe extern "C" fn(),
+    reserved: usize,
+}
+
+extern "C" {
+    fn WWDG();
+    fn PVD();
+}
+
+#[allow(unsafe_code)]
+#[link_section = ".vector_table.interrupts"]
+#[no_mangle]
+pub static __INTERRUPTS: [Vector; 3] = [
+    Vector { handler: WWDG },
+    Vector { reserved: 0 },
+    Vector { handler: PVD },
+];