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/litrs/src/string/mod.rs

@@ -0,0 +1,129 @@
+use std::{fmt, ops::Range};
+
+use crate::{
+    err::{perr, ParseErrorKind::*},
+    escape::{scan_raw_string, unescape_string},
+    parse::first_byte_or_empty,
+    Buffer, ParseError,
+};
+
+
+/// A string or raw string literal, e.g. `"foo"`, `"Grüße"` or `r#"a🦊c"d🦀f"#`.
+///
+/// See [the reference][ref] for more information.
+///
+/// [ref]: https://doc.rust-lang.org/reference/tokens.html#string-literals
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct StringLit<B: Buffer> {
+    /// The raw input.
+    raw: B,
+
+    /// The string value (with all escapes unescaped), or `None` if there were
+    /// no escapes. In the latter case, the string value is in `raw`.
+    value: Option<String>,
+
+    /// The number of hash signs in case of a raw string literal, or `None` if
+    /// it's not a raw string literal.
+    num_hashes: Option<u8>,
+
+    /// Start index of the suffix or `raw.len()` if there is no suffix.
+    start_suffix: usize,
+}
+
+impl<B: Buffer> StringLit<B> {
+    /// Parses the input as a (raw) string literal. Returns an error if the
+    /// input is invalid or represents a different kind of literal.
+    pub fn parse(input: B) -> Result<Self, ParseError> {
+        match first_byte_or_empty(&input)? {
+            b'r' | b'"' => {
+                let (value, num_hashes, start_suffix) = parse_impl(&input)?;
+                Ok(Self { raw: input, value, num_hashes, start_suffix })
+            }
+            _ => Err(perr(0, InvalidStringLiteralStart)),
+        }
+    }
+
+    /// Returns the string value this literal represents (where all escapes have
+    /// been turned into their respective values).
+    pub fn value(&self) -> &str {
+        self.value
+            .as_deref()
+            .unwrap_or(&self.raw[self.inner_range()])
+    }
+
+    /// Like `value` but returns a potentially owned version of the value.
+    ///
+    /// The return value is either `String` if `B = String`, or
+    /// `Cow<'a, str>` if `B = &'a str`.
+    pub fn into_value(self) -> B::Cow {
+        let inner_range = self.inner_range();
+        let Self { raw, value, .. } = self;
+        value
+            .map(B::Cow::from)
+            .unwrap_or_else(|| raw.cut(inner_range).into_cow())
+    }
+
+    /// The optional suffix. Returns `""` if the suffix is empty/does not exist.
+    pub fn suffix(&self) -> &str {
+        &(*self.raw)[self.start_suffix..]
+    }
+
+    /// Returns whether this literal is a raw string literal (starting with
+    /// `r`).
+    pub fn is_raw_string(&self) -> bool {
+        self.num_hashes.is_some()
+    }
+
+    /// Returns the raw input that was passed to `parse`.
+    pub fn raw_input(&self) -> &str {
+        &self.raw
+    }
+
+    /// Returns the raw input that was passed to `parse`, potentially owned.
+    pub fn into_raw_input(self) -> B {
+        self.raw
+    }
+
+    /// The range within `self.raw` that excludes the quotes and potential `r#`.
+    fn inner_range(&self) -> Range<usize> {
+        match self.num_hashes {
+            None => 1..self.start_suffix - 1,
+            Some(n) => 1 + n as usize + 1..self.start_suffix - n as usize - 1,
+        }
+    }
+}
+
+impl StringLit<&str> {
+    /// Makes a copy of the underlying buffer and returns the owned version of
+    /// `Self`.
+    pub fn into_owned(self) -> StringLit<String> {
+        StringLit {
+            raw: self.raw.to_owned(),
+            value: self.value,
+            num_hashes: self.num_hashes,
+            start_suffix: self.start_suffix,
+        }
+    }
+}
+
+impl<B: Buffer> fmt::Display for StringLit<B> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad(&self.raw)
+    }
+}
+
+/// Precondition: input has to start with either `"` or `r`.
+#[inline(never)]
+pub(crate) fn parse_impl(input: &str) -> Result<(Option<String>, Option<u8>, usize), ParseError> {
+    if input.starts_with('r') {
+        scan_raw_string(input, 1, true, true)
+            .map(|(hashes, start_suffix)| (None, Some(hashes), start_suffix))
+    } else {
+        unescape_string::<String>(input, 1, true, false, true)
+            .map(|(v, start_suffix)| (v, None, start_suffix))
+    }
+}
+
+
+#[cfg(test)]
+mod tests;