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/embassy-sync-0.5.0/src/blocking_mutex/raw.rs

@@ -0,0 +1,149 @@
+//! Mutex primitives.
+//!
+//! This module provides a trait for mutexes that can be used in different contexts.
+use core::marker::PhantomData;
+
+/// Raw mutex trait.
+///
+/// This mutex is "raw", which means it does not actually contain the protected data, it
+/// just implements the mutex mechanism. For most uses you should use [`super::Mutex`] instead,
+/// which is generic over a RawMutex and contains the protected data.
+///
+/// Note that, unlike other mutexes, implementations only guarantee no
+/// concurrent access from other threads: concurrent access from the current
+/// thread is allowed. For example, it's possible to lock the same mutex multiple times reentrantly.
+///
+/// Therefore, locking a `RawMutex` is only enough to guarantee safe shared (`&`) access
+/// to the data, it is not enough to guarantee exclusive (`&mut`) access.
+///
+/// # Safety
+///
+/// RawMutex implementations must ensure that, while locked, no other thread can lock
+/// the RawMutex concurrently.
+///
+/// Unsafe code is allowed to rely on this fact, so incorrect implementations will cause undefined behavior.
+pub unsafe trait RawMutex {
+    /// Create a new `RawMutex` instance.
+    ///
+    /// This is a const instead of a method to allow creating instances in const context.
+    const INIT: Self;
+
+    /// Lock this `RawMutex`.
+    fn lock<R>(&self, f: impl FnOnce() -> R) -> R;
+}
+
+/// A mutex that allows borrowing data across executors and interrupts.
+///
+/// # Safety
+///
+/// This mutex is safe to share between different executors and interrupts.
+pub struct CriticalSectionRawMutex {
+    _phantom: PhantomData<()>,
+}
+unsafe impl Send for CriticalSectionRawMutex {}
+unsafe impl Sync for CriticalSectionRawMutex {}
+
+impl CriticalSectionRawMutex {
+    /// Create a new `CriticalSectionRawMutex`.
+    pub const fn new() -> Self {
+        Self { _phantom: PhantomData }
+    }
+}
+
+unsafe impl RawMutex for CriticalSectionRawMutex {
+    const INIT: Self = Self::new();
+
+    fn lock<R>(&self, f: impl FnOnce() -> R) -> R {
+        critical_section::with(|_| f())
+    }
+}
+
+// ================
+
+/// A mutex that allows borrowing data in the context of a single executor.
+///
+/// # Safety
+///
+/// **This Mutex is only safe within a single executor.**
+pub struct NoopRawMutex {
+    _phantom: PhantomData<*mut ()>,
+}
+
+unsafe impl Send for NoopRawMutex {}
+
+impl NoopRawMutex {
+    /// Create a new `NoopRawMutex`.
+    pub const fn new() -> Self {
+        Self { _phantom: PhantomData }
+    }
+}
+
+unsafe impl RawMutex for NoopRawMutex {
+    const INIT: Self = Self::new();
+    fn lock<R>(&self, f: impl FnOnce() -> R) -> R {
+        f()
+    }
+}
+
+// ================
+
+#[cfg(any(cortex_m, feature = "std"))]
+mod thread_mode {
+    use super::*;
+
+    /// A "mutex" that only allows borrowing from thread mode.
+    ///
+    /// # Safety
+    ///
+    /// **This Mutex is only safe on single-core systems.**
+    ///
+    /// On multi-core systems, a `ThreadModeRawMutex` **is not sufficient** to ensure exclusive access.
+    pub struct ThreadModeRawMutex {
+        _phantom: PhantomData<()>,
+    }
+
+    unsafe impl Send for ThreadModeRawMutex {}
+    unsafe impl Sync for ThreadModeRawMutex {}
+
+    impl ThreadModeRawMutex {
+        /// Create a new `ThreadModeRawMutex`.
+        pub const fn new() -> Self {
+            Self { _phantom: PhantomData }
+        }
+    }
+
+    unsafe impl RawMutex for ThreadModeRawMutex {
+        const INIT: Self = Self::new();
+        fn lock<R>(&self, f: impl FnOnce() -> R) -> R {
+            assert!(in_thread_mode(), "ThreadModeMutex can only be locked from thread mode.");
+
+            f()
+        }
+    }
+
+    impl Drop for ThreadModeRawMutex {
+        fn drop(&mut self) {
+            // Only allow dropping from thread mode. Dropping calls drop on the inner `T`, so
+            // `drop` needs the same guarantees as `lock`. `ThreadModeMutex<T>` is Send even if
+            // T isn't, so without this check a user could create a ThreadModeMutex in thread mode,
+            // send it to interrupt context and drop it there, which would "send" a T even if T is not Send.
+            assert!(
+                in_thread_mode(),
+                "ThreadModeMutex can only be dropped from thread mode."
+            );
+
+            // Drop of the inner `T` happens after this.
+        }
+    }
+
+    pub(crate) fn in_thread_mode() -> bool {
+        #[cfg(feature = "std")]
+        return Some("main") == std::thread::current().name();
+
+        #[cfg(not(feature = "std"))]
+        // ICSR.VECTACTIVE == 0
+        return unsafe { (0xE000ED04 as *const u32).read_volatile() } & 0x1FF == 0;
+    }
+}
+#[cfg(any(cortex_m, feature = "std"))]
+pub use thread_mode::*;