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/examples/nightly.rs

@@ -0,0 +1,18 @@
+extern crate autocfg;
+
+fn main() {
+    // Normally, cargo will set `OUT_DIR` for build scripts.
+    let ac = autocfg::AutoCfg::with_dir("target").unwrap();
+
+    // When this feature was stabilized, it also renamed the method to
+    // `chunk_by`, so it's important to *use* the feature in your probe.
+    let code = r#"
+        #![feature(slice_group_by)]
+        pub fn probe(slice: &[i32]) -> impl Iterator<Item = &[i32]> {
+            slice.group_by(|a, b| a == b)
+        }
+    "#;
+    if ac.probe_raw(code).is_ok() {
+        autocfg::emit("has_slice_group_by");
+    }
+}