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/syn/src/span.rs

@@ -0,0 +1,63 @@
+use proc_macro2::extra::DelimSpan;
+use proc_macro2::{Delimiter, Group, Span, TokenStream};
+
+#[doc(hidden)]
+pub trait IntoSpans<S> {
+    fn into_spans(self) -> S;
+}
+
+impl IntoSpans<Span> for Span {
+    fn into_spans(self) -> Span {
+        self
+    }
+}
+
+impl IntoSpans<[Span; 1]> for Span {
+    fn into_spans(self) -> [Span; 1] {
+        [self]
+    }
+}
+
+impl IntoSpans<[Span; 2]> for Span {
+    fn into_spans(self) -> [Span; 2] {
+        [self, self]
+    }
+}
+
+impl IntoSpans<[Span; 3]> for Span {
+    fn into_spans(self) -> [Span; 3] {
+        [self, self, self]
+    }
+}
+
+impl IntoSpans<[Span; 1]> for [Span; 1] {
+    fn into_spans(self) -> [Span; 1] {
+        self
+    }
+}
+
+impl IntoSpans<[Span; 2]> for [Span; 2] {
+    fn into_spans(self) -> [Span; 2] {
+        self
+    }
+}
+
+impl IntoSpans<[Span; 3]> for [Span; 3] {
+    fn into_spans(self) -> [Span; 3] {
+        self
+    }
+}
+
+impl IntoSpans<DelimSpan> for Span {
+    fn into_spans(self) -> DelimSpan {
+        let mut group = Group::new(Delimiter::None, TokenStream::new());
+        group.set_span(self);
+        group.delim_span()
+    }
+}
+
+impl IntoSpans<DelimSpan> for DelimSpan {
+    fn into_spans(self) -> DelimSpan {
+        self
+    }
+}