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/xtensa-lx/src/macros.rs

@@ -0,0 +1,52 @@
+/// Macro to create a mutable reference to a statically allocated value
+///
+/// This macro returns a value with type `Option<&'static mut $ty>`.
+/// `Some($expr)` will be returned the first time the macro is executed; further
+/// calls will return `None`. To avoid `unwrap`ping a `None` variant the caller
+/// must ensure that the macro is called from a function that's executed at most
+/// once in the whole lifetime of the program.
+///
+/// # Example
+///
+/// ``` no_run
+/// use xtensa_lx::singleton;
+///
+/// fn main() {
+///     // OK if `main` is executed only once
+///     let x: &'static mut bool = singleton!(: bool = false).unwrap();
+///
+///     let y = alias();
+///     // BAD this second call to `alias` will definitively `panic!`
+///     let y_alias = alias();
+/// }
+///
+/// fn alias() -> &'static mut bool {
+///     singleton!(: bool = false).unwrap()
+/// }
+/// ```
+#[macro_export]
+macro_rules! singleton {
+    (: $ty:ty = $expr:expr) => {
+        $crate::interrupt::free(|_| {
+            static mut VAR: Option<$ty> = None;
+
+            #[allow(unsafe_code)]
+            let used = unsafe { VAR.is_some() };
+            if used {
+                None
+            } else {
+                let expr = $expr;
+
+                #[allow(unsafe_code)]
+                unsafe {
+                    VAR = Some(expr)
+                }
+
+                #[allow(unsafe_code)]
+                unsafe {
+                    VAR.as_mut()
+                }
+            }
+        })
+    };
+}