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/darling/examples/fallible_read.rs

@@ -0,0 +1,85 @@
+//! This example demonstrates techniques for performing custom error handling
+//! in a derive-input receiver.
+//!
+//! 1. Using `darling::Result` as a carrier to preserve the error for later display
+//! 1. Using `Result<T, syn::Meta>` to attempt a recovery in imperative code
+//! 1. Using the `map` darling meta-item to post-process a field before returning
+//! 1. Using the `and_then` darling meta-item to post-process the receiver before returning
+
+use darling::{FromDeriveInput, FromMeta};
+use syn::parse_str;
+
+#[derive(Debug, FromDeriveInput)]
+#[darling(attributes(my_trait), and_then = MyInputReceiver::autocorrect)]
+pub struct MyInputReceiver {
+    /// This field must be present and a string or else parsing will panic.
+    #[darling(map = MyInputReceiver::make_string_shouty)]
+    name: String,
+
+    /// If this field fails to parse, the struct can still be built; the field
+    /// will contain the error. The consuming struct can then decide if this
+    /// blocks code generation. If so, panic or fail in `and_then`.
+    frequency: darling::Result<i64>,
+
+    /// If this field fails to parse, the struct can still be built; the field
+    /// will contain an `Err` with the original `syn::Meta`. This can be used
+    /// for alternate parsing attempts before panicking.
+    amplitude: Result<u64, syn::Meta>,
+}
+
+impl MyInputReceiver {
+    /// This function will be called by `darling` _after_ it's finished parsing the
+    /// `name` field but before initializing `name` with the resulting value. It's
+    /// a good place for transforms that are easiest to express on already-built
+    /// types.
+    fn make_string_shouty(s: String) -> String {
+        s.to_uppercase()
+    }
+
+    /// This function will be called by `darling` _after_ it's finished parsing the
+    /// input but before returning to the caller. This is a good place to initialize
+    /// skipped fields or to perform corrections that don't lend themselves to being
+    /// done elsewhere.
+    fn autocorrect(self) -> darling::Result<Self> {
+        let Self {
+            name,
+            frequency,
+            amplitude,
+        } = self;
+
+        // Amplitude doesn't have a sign, so if we received a negative number then
+        // we'll go ahead and make it positive.
+        let amplitude = match amplitude {
+            Ok(amp) => amp,
+            Err(mi) => (i64::from_meta(&mi)?).unsigned_abs(),
+        };
+
+        Ok(Self {
+            name,
+            frequency,
+            amplitude: Ok(amplitude),
+        })
+    }
+}
+
+fn main() {
+    let input = r#"#[derive(MyTrait)]
+#[my_trait(name="Jon", amplitude = "-1", frequency = 1)]
+pub struct Foo;"#;
+
+    let parsed = parse_str(input).unwrap();
+    let receiver = MyInputReceiver::from_derive_input(&parsed).unwrap();
+
+    println!(
+        r#"
+INPUT:
+
+{}
+
+PARSED AS:
+
+{:?}
+    "#,
+        input, receiver
+    );
+}