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/alignment.rs

@@ -0,0 +1,33 @@
+//! This is not an example; this is a link-pass test
+
+#![deny(warnings)]
+#![no_main]
+#![no_std]
+
+extern crate cortex_m_rt as rt;
+extern crate panic_halt;
+
+use core::ptr;
+
+use rt::entry;
+
+static mut BSS1: u16 = 0;
+static mut BSS2: u8 = 0;
+static mut DATA1: u8 = 1;
+static mut DATA2: u16 = 1;
+static RODATA1: &[u8; 3] = b"012";
+static RODATA2: &[u8; 2] = b"34";
+
+#[entry]
+fn main() -> ! {
+    unsafe {
+        let _bss1 = ptr::read_volatile(ptr::addr_of!(BSS1));
+        let _bss2 = ptr::read_volatile(ptr::addr_of!(BSS2));
+        let _data1 = ptr::read_volatile(ptr::addr_of!(DATA1));
+        let _data2 = ptr::read_volatile(ptr::addr_of!(DATA2));
+        let _rodata1 = ptr::read_volatile(ptr::addr_of!(RODATA1));
+        let _rodata2 = ptr::read_volatile(ptr::addr_of!(RODATA2));
+    }
+
+    loop {}
+}