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/hashbrown/benches/insert_unique_unchecked.rs

@@ -0,0 +1,34 @@
+//! Compare `insert` and `insert_unique_unchecked` operations performance.
+
+#![feature(test)]
+
+extern crate test;
+
+use hashbrown::HashMap;
+use test::Bencher;
+
+#[bench]
+fn insert(b: &mut Bencher) {
+    let keys: Vec<String> = (0..1000).map(|i| format!("xxxx{}yyyy", i)).collect();
+    b.iter(|| {
+        let mut m = HashMap::with_capacity(1000);
+        for k in &keys {
+            m.insert(k, k);
+        }
+        m
+    });
+}
+
+#[bench]
+fn insert_unique_unchecked(b: &mut Bencher) {
+    let keys: Vec<String> = (0..1000).map(|i| format!("xxxx{}yyyy", i)).collect();
+    b.iter(|| {
+        let mut m = HashMap::with_capacity(1000);
+        for k in &keys {
+            unsafe {
+                m.insert_unique_unchecked(k, k);
+            }
+        }
+        m
+    });
+}