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/autocfg/src/error.rs

@@ -0,0 +1,81 @@
+use std::error;
+use std::fmt;
+use std::io;
+use std::num;
+use std::process;
+use std::str;
+
+/// A common error type for the `autocfg` crate.
+#[derive(Debug)]
+pub struct Error {
+    kind: ErrorKind,
+}
+
+impl error::Error for Error {
+    fn description(&self) -> &str {
+        "AutoCfg error"
+    }
+
+    fn cause(&self) -> Option<&error::Error> {
+        match self.kind {
+            ErrorKind::Io(ref e) => Some(e),
+            ErrorKind::Num(ref e) => Some(e),
+            ErrorKind::Utf8(ref e) => Some(e),
+            ErrorKind::Process(_) | ErrorKind::Other(_) => None,
+        }
+    }
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        match self.kind {
+            ErrorKind::Io(ref e) => e.fmt(f),
+            ErrorKind::Num(ref e) => e.fmt(f),
+            ErrorKind::Utf8(ref e) => e.fmt(f),
+            ErrorKind::Process(ref status) => {
+                // Same message as the newer `ExitStatusError`
+                write!(f, "process exited unsuccessfully: {}", status)
+            }
+            ErrorKind::Other(s) => s.fmt(f),
+        }
+    }
+}
+
+#[derive(Debug)]
+enum ErrorKind {
+    Io(io::Error),
+    Num(num::ParseIntError),
+    Process(process::ExitStatus),
+    Utf8(str::Utf8Error),
+    Other(&'static str),
+}
+
+pub fn from_exit(status: process::ExitStatus) -> Error {
+    Error {
+        kind: ErrorKind::Process(status),
+    }
+}
+
+pub fn from_io(e: io::Error) -> Error {
+    Error {
+        kind: ErrorKind::Io(e),
+    }
+}
+
+pub fn from_num(e: num::ParseIntError) -> Error {
+    Error {
+        kind: ErrorKind::Num(e),
+    }
+}
+
+pub fn from_utf8(e: str::Utf8Error) -> Error {
+    Error {
+        kind: ErrorKind::Utf8(e),
+    }
+}
+
+pub fn from_str(s: &'static str) -> Error {
+    Error {
+        kind: ErrorKind::Other(s),
+    }
+}