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/override-exception.rs

@@ -0,0 +1,29 @@
+//! How to override the hard fault exception handler and the default exception handler
+
+#![deny(warnings)]
+#![no_main]
+#![no_std]
+
+extern crate cortex_m;
+extern crate cortex_m_rt as rt;
+extern crate panic_halt;
+
+use cortex_m::asm;
+use rt::{entry, exception, ExceptionFrame};
+
+#[entry]
+fn main() -> ! {
+    loop {}
+}
+
+#[exception]
+unsafe fn DefaultHandler(_irqn: i16) {
+    asm::bkpt();
+}
+
+#[exception]
+unsafe fn HardFault(_ef: &ExceptionFrame) -> ! {
+    asm::bkpt();
+
+    loop {}
+}