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/serde.rs

@@ -0,0 +1,65 @@
+#![cfg(feature = "serde")]
+
+use core::hash::BuildHasherDefault;
+use fnv::FnvHasher;
+use hashbrown::{HashMap, HashSet};
+use serde_test::{assert_tokens, Token};
+
+// We use FnvHash for this test because we rely on the ordering
+type FnvHashMap<K, V> = HashMap<K, V, BuildHasherDefault<FnvHasher>>;
+type FnvHashSet<T> = HashSet<T, BuildHasherDefault<FnvHasher>>;
+
+#[test]
+fn map_serde_tokens_empty() {
+    let map = FnvHashMap::<char, u32>::default();
+
+    assert_tokens(&map, &[Token::Map { len: Some(0) }, Token::MapEnd]);
+}
+
+#[test]
+fn map_serde_tokens() {
+    let mut map = FnvHashMap::default();
+    map.insert('b', 20);
+    map.insert('a', 10);
+    map.insert('c', 30);
+
+    assert_tokens(
+        &map,
+        &[
+            Token::Map { len: Some(3) },
+            Token::Char('a'),
+            Token::I32(10),
+            Token::Char('c'),
+            Token::I32(30),
+            Token::Char('b'),
+            Token::I32(20),
+            Token::MapEnd,
+        ],
+    );
+}
+
+#[test]
+fn set_serde_tokens_empty() {
+    let set = FnvHashSet::<u32>::default();
+
+    assert_tokens(&set, &[Token::Seq { len: Some(0) }, Token::SeqEnd]);
+}
+
+#[test]
+fn set_serde_tokens() {
+    let mut set = FnvHashSet::default();
+    set.insert(20);
+    set.insert(10);
+    set.insert(30);
+
+    assert_tokens(
+        &set,
+        &[
+            Token::Seq { len: Some(3) },
+            Token::I32(30),
+            Token::I32(20),
+            Token::I32(10),
+            Token::SeqEnd,
+        ],
+    );
+}