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/rustc_version/src/errors.rs

@@ -0,0 +1,79 @@
+use std::{self, error, fmt, io, str};
+use semver::{self, Identifier};
+
+/// The error type for this crate.
+#[derive(Debug)]
+pub enum Error {
+    /// An error ocurrend when executing the `rustc` command.
+    CouldNotExecuteCommand(io::Error),
+    /// The output of `rustc -vV` was not valid utf-8.
+    Utf8Error(str::Utf8Error),
+    /// The output of `rustc -vV` was not in the expected format.
+    UnexpectedVersionFormat,
+    /// An error ocurred in parsing a `VersionReq`.
+    ReqParseError(semver::ReqParseError),
+    /// An error ocurred in parsing the semver.
+    SemVerError(semver::SemVerError),
+    /// The pre-release tag is unknown.
+    UnknownPreReleaseTag(Identifier),
+}
+use Error::*;
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        use std::error::Error;
+        match *self {
+            CouldNotExecuteCommand(ref e) => write!(f, "{}: {}", self.description(), e),
+            Utf8Error(_) => write!(f, "{}", self.description()),
+            UnexpectedVersionFormat => write!(f, "{}", self.description()),
+            ReqParseError(ref e) => write!(f, "{}: {}", self.description(), e),
+            SemVerError(ref e) => write!(f, "{}: {}", self.description(), e),
+            UnknownPreReleaseTag(ref i) => write!(f, "{}: {}", self.description(), i),
+        }
+    }
+}
+
+impl error::Error for Error {
+    fn cause(&self) -> Option<&error::Error> {
+        match *self {
+            CouldNotExecuteCommand(ref e) => Some(e),
+            Utf8Error(ref e) => Some(e),
+            UnexpectedVersionFormat => None,
+            ReqParseError(ref e) => Some(e),
+            SemVerError(ref e) => Some(e),
+            UnknownPreReleaseTag(_) => None,
+        }
+    }
+
+    fn description(&self) -> &str {
+        match *self {
+            CouldNotExecuteCommand(_) => "could not execute command",
+            Utf8Error(_) => "invalid UTF-8 output from `rustc -vV`",
+            UnexpectedVersionFormat => "unexpected `rustc -vV` format",
+            ReqParseError(_) => "error parsing version requirement",
+            SemVerError(_) => "error parsing version",
+            UnknownPreReleaseTag(_) => "unknown pre-release tag",
+        }
+    }
+}
+
+macro_rules! impl_from {
+    ($($err_ty:ty => $variant:ident),* $(,)*) => {
+        $(
+            impl From<$err_ty> for Error {
+                fn from(e: $err_ty) -> Error {
+                    Error::$variant(e)
+                }
+            }
+        )*
+    }
+}
+
+impl_from! {
+    str::Utf8Error => Utf8Error,
+    semver::SemVerError => SemVerError,
+    semver::ReqParseError => ReqParseError,
+}
+
+/// The result type for this crate.
+pub type Result<T> = std::result::Result<T, Error>;