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/parser_ast.rs

@@ -0,0 +1,100 @@
+use std::fmt;
+use std::fmt::{Debug, Display, Formatter};
+
+use std::str::FromStr;
+
+use winnow::prelude::*;
+use winnow::Result;
+use winnow::{
+    ascii::{digit1 as digits, multispace0 as multispaces},
+    combinator::alt,
+    combinator::delimited,
+    combinator::repeat,
+    token::one_of,
+};
+
+#[derive(Debug, Clone)]
+pub(crate) enum Expr {
+    Value(i64),
+    Add(Box<Expr>, Box<Expr>),
+    Sub(Box<Expr>, Box<Expr>),
+    Mul(Box<Expr>, Box<Expr>),
+    Div(Box<Expr>, Box<Expr>),
+    Paren(Box<Expr>),
+}
+
+impl Expr {
+    pub(crate) fn eval(&self) -> i64 {
+        match self {
+            Self::Value(v) => *v,
+            Self::Add(lhs, rhs) => lhs.eval() + rhs.eval(),
+            Self::Sub(lhs, rhs) => lhs.eval() - rhs.eval(),
+            Self::Mul(lhs, rhs) => lhs.eval() * rhs.eval(),
+            Self::Div(lhs, rhs) => lhs.eval() / rhs.eval(),
+            Self::Paren(expr) => expr.eval(),
+        }
+    }
+}
+
+impl Display for Expr {
+    fn fmt(&self, format: &mut Formatter<'_>) -> fmt::Result {
+        use Expr::{Add, Div, Mul, Paren, Sub, Value};
+        match *self {
+            Value(val) => write!(format, "{val}"),
+            Add(ref left, ref right) => write!(format, "{left} + {right}"),
+            Sub(ref left, ref right) => write!(format, "{left} - {right}"),
+            Mul(ref left, ref right) => write!(format, "{left} * {right}"),
+            Div(ref left, ref right) => write!(format, "{left} / {right}"),
+            Paren(ref expr) => write!(format, "({expr})"),
+        }
+    }
+}
+
+pub(crate) fn expr(i: &mut &str) -> Result<Expr> {
+    let init = term.parse_next(i)?;
+
+    repeat(0.., (one_of(['+', '-']), term))
+        .fold(
+            move || init.clone(),
+            |acc, (op, val): (char, Expr)| {
+                if op == '+' {
+                    Expr::Add(Box::new(acc), Box::new(val))
+                } else {
+                    Expr::Sub(Box::new(acc), Box::new(val))
+                }
+            },
+        )
+        .parse_next(i)
+}
+
+pub(crate) fn term(i: &mut &str) -> Result<Expr> {
+    let init = factor.parse_next(i)?;
+
+    repeat(0.., (one_of(['*', '/']), factor))
+        .fold(
+            move || init.clone(),
+            |acc, (op, val): (char, Expr)| {
+                if op == '*' {
+                    Expr::Mul(Box::new(acc), Box::new(val))
+                } else {
+                    Expr::Div(Box::new(acc), Box::new(val))
+                }
+            },
+        )
+        .parse_next(i)
+}
+
+pub(crate) fn factor(i: &mut &str) -> Result<Expr> {
+    delimited(
+        multispaces,
+        alt((digits.try_map(FromStr::from_str).map(Expr::Value), parens)),
+        multispaces,
+    )
+    .parse_next(i)
+}
+
+fn parens(i: &mut &str) -> Result<Expr> {
+    delimited("(", expr, ")")
+        .map(|e| Expr::Paren(Box::new(e)))
+        .parse_next(i)
+}