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/src/interrupt.rs

@@ -0,0 +1,73 @@
+//! Interrupts
+
+pub use bare_metal::{CriticalSection, Mutex, Nr};
+
+/// Trait for enums of external interrupt numbers.
+///
+/// This trait should be implemented by a peripheral access crate (PAC)
+/// on its enum of available external interrupts for a specific device.
+/// Each variant must convert to a u16 of its interrupt number,
+/// which is its exception number - 16.
+///
+/// # Safety
+///
+/// This trait must only be implemented on enums of device interrupts. Each
+/// enum variant must represent a distinct value (no duplicates are permitted),
+/// and must always return the same value (do not change at runtime).
+///
+/// These requirements ensure safe nesting of critical sections.
+pub unsafe trait InterruptNumber: Copy {
+    /// Return the interrupt number associated with this variant.
+    ///
+    /// See trait documentation for safety requirements.
+    fn number(self) -> u16;
+}
+
+/// Implement InterruptNumber for the old bare_metal::Nr trait.
+/// This implementation is for backwards compatibility only and will be removed in cortex-m 0.8.
+unsafe impl<T: Nr + Copy> InterruptNumber for T {
+    #[inline]
+    fn number(self) -> u16 {
+        self.nr() as u16
+    }
+}
+
+/// Disables all interrupts
+#[inline]
+pub fn disable() {
+    call_asm!(__cpsid());
+}
+
+/// Enables all the interrupts
+///
+/// # Safety
+///
+/// - Do not call this function inside an `interrupt::free` critical section
+#[inline]
+pub unsafe fn enable() {
+    call_asm!(__cpsie());
+}
+
+/// Execute closure `f` in an interrupt-free context.
+///
+/// This as also known as a "critical section".
+#[inline]
+pub fn free<F, R>(f: F) -> R
+where
+    F: FnOnce(&CriticalSection) -> R,
+{
+    let primask = crate::register::primask::read();
+
+    // disable interrupts
+    disable();
+
+    let r = f(unsafe { &CriticalSection::new() });
+
+    // If the interrupts were active before our `disable` call, then re-enable
+    // them. Otherwise, keep them disabled
+    if primask.is_active() {
+        unsafe { enable() }
+    }
+
+    r
+}