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/critical-section/src/std.rs

@@ -0,0 +1,87 @@
+use std::cell::Cell;
+use std::mem::MaybeUninit;
+use std::sync::{Mutex, MutexGuard};
+
+static GLOBAL_MUTEX: Mutex<()> = Mutex::new(());
+
+// This is initialized if a thread has acquired the CS, uninitialized otherwise.
+static mut GLOBAL_GUARD: MaybeUninit<MutexGuard<'static, ()>> = MaybeUninit::uninit();
+
+std::thread_local!(static IS_LOCKED: Cell<bool> = Cell::new(false));
+
+struct StdCriticalSection;
+crate::set_impl!(StdCriticalSection);
+
+unsafe impl crate::Impl for StdCriticalSection {
+    unsafe fn acquire() -> bool {
+        // Allow reentrancy by checking thread local state
+        IS_LOCKED.with(|l| {
+            if l.get() {
+                // CS already acquired in the current thread.
+                return true;
+            }
+
+            // Note: it is fine to set this flag *before* acquiring the mutex because it's thread local.
+            // No other thread can see its value, there's no potential for races.
+            // This way, we hold the mutex for slightly less time.
+            l.set(true);
+
+            // Not acquired in the current thread, acquire it.
+            let guard = match GLOBAL_MUTEX.lock() {
+                Ok(guard) => guard,
+                Err(err) => {
+                    // Ignore poison on the global mutex in case a panic occurred
+                    // while the mutex was held.
+                    err.into_inner()
+                }
+            };
+            GLOBAL_GUARD.write(guard);
+
+            false
+        })
+    }
+
+    unsafe fn release(nested_cs: bool) {
+        if !nested_cs {
+            // SAFETY: As per the acquire/release safety contract, release can only be called
+            // if the critical section is acquired in the current thread,
+            // in which case we know the GLOBAL_GUARD is initialized.
+            //
+            // We have to `assume_init_read` then drop instead of `assume_init_drop` because:
+            // - drop requires exclusive access (&mut) to the contents
+            // - mutex guard drop first unlocks the mutex, then returns. In between those, there's a brief
+            //   moment where the mutex is unlocked but a `&mut` to the contents exists.
+            // - During this moment, another thread can go and use GLOBAL_GUARD, causing `&mut` aliasing.
+            #[allow(let_underscore_lock)]
+            let _ = GLOBAL_GUARD.assume_init_read();
+
+            // Note: it is fine to clear this flag *after* releasing the mutex because it's thread local.
+            // No other thread can see its value, there's no potential for races.
+            // This way, we hold the mutex for slightly less time.
+            IS_LOCKED.with(|l| l.set(false));
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::thread;
+
+    use crate as critical_section;
+
+    #[cfg(feature = "std")]
+    #[test]
+    #[should_panic(expected = "Not a PoisonError!")]
+    fn reusable_after_panic() {
+        let _ = thread::spawn(|| {
+            critical_section::with(|_| {
+                panic!("Boom!");
+            })
+        })
+        .join();
+
+        critical_section::with(|_| {
+            panic!("Not a PoisonError!");
+        })
+    }
+}