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/version.rs

@@ -0,0 +1,65 @@
+use std::process::Command;
+use std::str;
+
+use super::{error, Error};
+
+/// A version structure for making relative comparisons.
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Version {
+    major: usize,
+    minor: usize,
+    patch: usize,
+}
+
+impl Version {
+    /// Creates a `Version` instance for a specific `major.minor.patch` version.
+    pub fn new(major: usize, minor: usize, patch: usize) -> Self {
+        Version {
+            major: major,
+            minor: minor,
+            patch: patch,
+        }
+    }
+
+    pub fn from_command(command: &mut Command) -> Result<Self, Error> {
+        // Get rustc's verbose version
+        let output = try!(command
+            .args(&["--version", "--verbose"])
+            .output()
+            .map_err(error::from_io));
+        if !output.status.success() {
+            return Err(error::from_str("could not execute rustc"));
+        }
+        let output = try!(str::from_utf8(&output.stdout).map_err(error::from_utf8));
+
+        // Find the release line in the verbose version output.
+        let release = match output.lines().find(|line| line.starts_with("release: ")) {
+            Some(line) => &line["release: ".len()..],
+            None => return Err(error::from_str("could not find rustc release")),
+        };
+
+        // Strip off any extra channel info, e.g. "-beta.N", "-nightly"
+        let version = match release.find('-') {
+            Some(i) => &release[..i],
+            None => release,
+        };
+
+        // Split the version into semver components.
+        let mut iter = version.splitn(3, '.');
+        let major = try!(iter
+            .next()
+            .ok_or_else(|| error::from_str("missing major version")));
+        let minor = try!(iter
+            .next()
+            .ok_or_else(|| error::from_str("missing minor version")));
+        let patch = try!(iter
+            .next()
+            .ok_or_else(|| error::from_str("missing patch version")));
+
+        Ok(Version::new(
+            try!(major.parse().map_err(error::from_num)),
+            try!(minor.parse().map_err(error::from_num)),
+            try!(patch.parse().map_err(error::from_num)),
+        ))
+    }
+}