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/warnings.rs

@@ -0,0 +1,51 @@
+//! Tests that a crate can still build with all warnings enabled.
+//!
+//! The code generated by the `cortex-m-rt` macros might need to manually
+//! `#[allow]` some of them (even though Rust does that by default for a few
+//! warnings too).
+
+#![no_std]
+#![no_main]
+#![deny(warnings, missing_docs, rust_2018_idioms)]
+
+extern crate cortex_m_rt;
+extern crate panic_halt;
+
+use cortex_m_rt::{entry, exception, interrupt, pre_init, ExceptionFrame};
+
+#[allow(non_camel_case_types)]
+enum interrupt {
+    INT,
+}
+
+extern "C" {
+    fn INT();
+}
+
+#[repr(C)]
+union Vector {
+    #[allow(dead_code)]
+    handler: unsafe extern "C" fn(),
+}
+
+#[link_section = ".vector_table.interrupts"]
+#[no_mangle]
+#[used]
+static __INTERRUPTS: [Vector; 1] = [Vector { handler: INT }];
+
+/// Dummy interrupt.
+#[interrupt]
+fn INT() {}
+
+#[exception]
+fn HardFault(_eh: &ExceptionFrame) -> ! {
+    loop {}
+}
+
+#[entry]
+fn main() -> ! {
+    loop {}
+}
+
+#[pre_init]
+unsafe fn pre_init() {}