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/rustversion/src/error.rs

@@ -0,0 +1,56 @@
+use proc_macro::{Delimiter, Group, Ident, Literal, Punct, Spacing, Span, TokenStream, TokenTree};
+use std::fmt::Display;
+use std::iter::FromIterator;
+
+pub type Result<T, E = Error> = std::result::Result<T, E>;
+
+pub struct Error {
+    begin: Span,
+    end: Span,
+    msg: String,
+}
+
+impl Error {
+    pub fn new(span: Span, msg: impl Display) -> Self {
+        Self::new2(span, span, msg)
+    }
+
+    pub fn new2(begin: Span, end: Span, msg: impl Display) -> Self {
+        Error {
+            begin,
+            end,
+            msg: msg.to_string(),
+        }
+    }
+
+    pub fn group(group: Group, msg: impl Display) -> Self {
+        let mut iter = group.stream().into_iter();
+        let delimiter = group.span();
+        let begin = iter.next().map_or(delimiter, |t| t.span());
+        let end = iter.last().map_or(begin, |t| t.span());
+        Self::new2(begin, end, msg)
+    }
+
+    pub fn into_compile_error(self) -> TokenStream {
+        // compile_error! { $msg }
+        TokenStream::from_iter(vec![
+            TokenTree::Ident(Ident::new("compile_error", self.begin)),
+            TokenTree::Punct({
+                let mut punct = Punct::new('!', Spacing::Alone);
+                punct.set_span(self.begin);
+                punct
+            }),
+            TokenTree::Group({
+                let mut group = Group::new(Delimiter::Brace, {
+                    TokenStream::from_iter(vec![TokenTree::Literal({
+                        let mut string = Literal::string(&self.msg);
+                        string.set_span(self.end);
+                        string
+                    })])
+                });
+                group.set_span(self.end);
+                group
+            }),
+        ])
+    }
+}