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/tests/equivalent_trait.rs

@@ -0,0 +1,53 @@
+use hashbrown::Equivalent;
+use hashbrown::HashMap;
+
+use std::hash::Hash;
+
+#[derive(Debug, Hash)]
+pub struct Pair<A, B>(pub A, pub B);
+
+impl<A, B, C, D> PartialEq<(A, B)> for Pair<C, D>
+where
+    C: PartialEq<A>,
+    D: PartialEq<B>,
+{
+    fn eq(&self, rhs: &(A, B)) -> bool {
+        self.0 == rhs.0 && self.1 == rhs.1
+    }
+}
+
+impl<A, B, X> Equivalent<X> for Pair<A, B>
+where
+    Pair<A, B>: PartialEq<X>,
+    A: Hash + Eq,
+    B: Hash + Eq,
+{
+    fn equivalent(&self, other: &X) -> bool {
+        *self == *other
+    }
+}
+
+#[test]
+fn test_lookup() {
+    let s = String::from;
+    let mut map = HashMap::new();
+    map.insert((s("a"), s("b")), 1);
+    map.insert((s("a"), s("x")), 2);
+
+    assert!(map.contains_key(&Pair("a", "b")));
+    assert!(!map.contains_key(&Pair("b", "a")));
+}
+
+#[test]
+fn test_string_str() {
+    let s = String::from;
+    let mut map = HashMap::new();
+    map.insert(s("a"), 1);
+    map.insert(s("b"), 2);
+    map.insert(s("x"), 3);
+    map.insert(s("y"), 4);
+
+    assert!(map.contains_key("a"));
+    assert!(!map.contains_key("z"));
+    assert_eq!(map.remove("b"), Some(2));
+}