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/winnow/examples/arithmetic/main.rs

@@ -0,0 +1,89 @@
+use winnow::prelude::*;
+
+mod parser;
+mod parser_ast;
+mod parser_lexer;
+#[cfg(test)]
+mod test_parser;
+#[cfg(test)]
+mod test_parser_ast;
+#[cfg(test)]
+mod test_parser_lexer;
+
+fn main() -> Result<(), lexopt::Error> {
+    let args = Args::parse()?;
+
+    let input = args.input.as_deref().unwrap_or("1 + 1");
+    if let Err(err) = calc(input, args.implementation) {
+        println!("FAILED");
+        println!("{err}");
+    }
+
+    Ok(())
+}
+
+fn calc(
+    input: &str,
+    imp: Impl,
+) -> Result<(), winnow::error::ParseError<&str, winnow::error::ContextError>> {
+    println!("{input} =");
+    match imp {
+        Impl::Eval => {
+            let result = parser::expr.parse(input)?;
+            println!("  {result}");
+        }
+        Impl::Ast => {
+            let result = parser_ast::expr.parse(input)?;
+            println!("  {:#?}={}", result, result.eval());
+        }
+        Impl::Lexer => {
+            let tokens = parser_lexer::tokens.parse(input)?;
+            println!("  {tokens:#?}");
+            let tokens = parser_lexer::Tokens::new(&tokens);
+            let result = parser_lexer::expr.parse(tokens).unwrap();
+            println!("  {:#?}={}", result, result.eval());
+        }
+    }
+    Ok(())
+}
+
+#[derive(Default)]
+struct Args {
+    input: Option<String>,
+    implementation: Impl,
+}
+
+#[derive(Default)]
+enum Impl {
+    #[default]
+    Eval,
+    Ast,
+    Lexer,
+}
+
+impl Args {
+    fn parse() -> Result<Self, lexopt::Error> {
+        use lexopt::prelude::*;
+
+        let mut res = Args::default();
+
+        let mut args = lexopt::Parser::from_env();
+        while let Some(arg) = args.next()? {
+            match arg {
+                Long("impl") => {
+                    res.implementation = args.value()?.parse_with(|s| match s {
+                        "eval" => Ok(Impl::Eval),
+                        "ast" => Ok(Impl::Ast),
+                        "lexer" => Ok(Impl::Lexer),
+                        _ => Err("expected `eval`, `ast`"),
+                    })?;
+                }
+                Value(input) => {
+                    res.input = Some(input.string()?);
+                }
+                _ => return Err(arg.unexpected()),
+            }
+        }
+        Ok(res)
+    }
+}