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/hard-fault-trampoline.rs

@@ -0,0 +1,31 @@
+//! This is not an example; this is a linker test that ensures
+//! that the jump from HardFault to _HardFault doesn't exceed
+//! the 2kB limit of the branch instruction.
+
+#![deny(warnings)]
+#![no_main]
+#![no_std]
+
+extern crate cortex_m_rt;
+extern crate panic_halt;
+
+use core::arch::asm;
+use cortex_m_rt::{entry, exception, ExceptionFrame};
+
+// This defines both `HardFault` and `_HardFault`. Both should have
+// link_section attributes placing them at the end of the .text section,
+// close to each other. If one of them is missing that attribute, they
+// will end up separated by `foo`, which will make the linker fail.
+#[exception(trampoline = true)]
+unsafe fn HardFault(_ef: &ExceptionFrame) -> ! {
+    loop {}
+}
+
+#[entry]
+fn foo() -> ! {
+    unsafe {
+        // 2kB of NOP instructions to make the function artificially larger
+        asm!(".fill 1024,2,0xbf00",);
+    }
+    loop {}
+}