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/qemu.rs

@@ -0,0 +1,34 @@
+#![no_main]
+#![no_std]
+
+use core::{
+    fmt::Write,
+    sync::atomic::{AtomicU32, Ordering},
+};
+
+static DATA_VAL: AtomicU32 = AtomicU32::new(1234);
+
+static BSS_VAL: AtomicU32 = AtomicU32::new(0);
+
+#[cortex_m_rt::entry]
+fn main() -> ! {
+    let x = 42;
+
+    loop {
+        let mut hstdout = cortex_m_semihosting::hio::hstdout().unwrap();
+        // check that .data and .bss were initialised OK
+        if DATA_VAL.load(Ordering::Relaxed) == 1234 && BSS_VAL.load(Ordering::Relaxed) == 0 {
+            _ = writeln!(hstdout, "x = {}", x);
+        }
+        cortex_m_semihosting::debug::exit(cortex_m_semihosting::debug::EXIT_SUCCESS);
+    }
+}
+
+// Define a panic handler that uses semihosting to exit immediately,
+// so that any panics cause qemu to quit instead of hang.
+#[panic_handler]
+fn panic(_: &core::panic::PanicInfo) -> ! {
+    loop {
+        cortex_m_semihosting::debug::exit(cortex_m_semihosting::debug::EXIT_FAILURE);
+    }
+}