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/proc-macro2/src/location.rs

@@ -0,0 +1,29 @@
+use core::cmp::Ordering;
+
+/// A line-column pair representing the start or end of a `Span`.
+///
+/// This type is semver exempt and not exposed by default.
+#[cfg_attr(docsrs, doc(cfg(feature = "span-locations")))]
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub struct LineColumn {
+    /// The 1-indexed line in the source file on which the span starts or ends
+    /// (inclusive).
+    pub line: usize,
+    /// The 0-indexed column (in UTF-8 characters) in the source file on which
+    /// the span starts or ends (inclusive).
+    pub column: usize,
+}
+
+impl Ord for LineColumn {
+    fn cmp(&self, other: &Self) -> Ordering {
+        self.line
+            .cmp(&other.line)
+            .then(self.column.cmp(&other.column))
+    }
+}
+
+impl PartialOrd for LineColumn {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}