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.rs

@@ -0,0 +1,68 @@
+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,
+};
+
+// Parser definition
+
+pub(crate) fn expr(i: &mut &str) -> Result<i64> {
+    let init = term.parse_next(i)?;
+
+    repeat(0.., (one_of(['+', '-']), term))
+        .fold(
+            move || init,
+            |acc, (op, val): (char, i64)| {
+                if op == '+' {
+                    acc + val
+                } else {
+                    acc - val
+                }
+            },
+        )
+        .parse_next(i)
+}
+
+// We read an initial factor and for each time we find
+// a * or / operator followed by another factor, we do
+// the math by folding everything
+pub(crate) fn term(i: &mut &str) -> Result<i64> {
+    let init = factor.parse_next(i)?;
+
+    repeat(0.., (one_of(['*', '/']), factor))
+        .fold(
+            move || init,
+            |acc, (op, val): (char, i64)| {
+                if op == '*' {
+                    acc * val
+                } else {
+                    acc / val
+                }
+            },
+        )
+        .parse_next(i)
+}
+
+// We transform an integer string into a i64, ignoring surrounding whitespace
+// We look for a digit suite, and try to convert it.
+// If either str::from_utf8 or FromStr::from_str fail,
+// we fallback to the parens parser defined above
+pub(crate) fn factor(i: &mut &str) -> Result<i64> {
+    delimited(
+        multispaces,
+        alt((digits.try_map(FromStr::from_str), parens)),
+        multispaces,
+    )
+    .parse_next(i)
+}
+
+// We parse any expr surrounded by parens, ignoring all whitespace around those
+fn parens(i: &mut &str) -> Result<i64> {
+    delimited('(', expr, ')').parse_next(i)
+}