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/string/main.rs

@@ -0,0 +1,70 @@
+//! This example shows an example of how to parse an escaped string. The
+//! rules for the string are similar to JSON and rust. A string is:
+//!
+//! - Enclosed by double quotes
+//! - Can contain any raw unescaped code point besides \ and "
+//! - Matches the following escape sequences: \b, \f, \n, \r, \t, \", \\, \/
+//! - Matches code points like Rust: \u{XXXX}, where XXXX can be up to 6
+//!   hex characters
+//! - an escape followed by whitespace consumes all whitespace between the
+//!   escape and the next non-whitespace character
+
+#![cfg(feature = "alloc")]
+
+mod parser;
+
+use winnow::prelude::*;
+
+fn main() -> Result<(), lexopt::Error> {
+    let args = Args::parse()?;
+
+    let data = args.input.as_deref().unwrap_or("\"abc\"");
+    let result = parser::parse_string::<()>.parse(data);
+    match result {
+        Ok(data) => println!("{data}"),
+        Err(err) => println!("{err:?}"),
+    }
+
+    Ok(())
+}
+
+#[derive(Default)]
+struct Args {
+    input: Option<String>,
+}
+
+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 {
+                Value(input) => {
+                    res.input = Some(input.string()?);
+                }
+                _ => return Err(arg.unexpected()),
+            }
+        }
+        Ok(res)
+    }
+}
+
+#[test]
+fn simple() {
+    let data = "\"abc\"";
+    let result = parser::parse_string::<()>.parse(data);
+    assert_eq!(result, Ok(String::from("abc")));
+}
+
+#[test]
+fn escaped() {
+    let data = "\"tab:\\tafter tab, newline:\\nnew line, quote: \\\", emoji: \\u{1F602}, newline:\\nescaped whitespace: \\    abc\"";
+    let result = parser::parse_string::<()>.parse(data);
+    assert_eq!(
+        result,
+        Ok(String::from("tab:\tafter tab, newline:\nnew line, quote: \", emoji: 😂, newline:\nescaped whitespace: abc"))
+    );
+}