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/hasher.rs

@@ -0,0 +1,65 @@
+//! Sanity check that alternate hashers work correctly.
+
+#![cfg(not(miri))] // FIXME: takes too long
+
+use hashbrown::HashSet;
+use std::hash::{BuildHasher, BuildHasherDefault, Hasher};
+
+fn check<S: BuildHasher + Default>() {
+    let range = 0..1_000;
+
+    let mut set = HashSet::<i32, S>::default();
+    set.extend(range.clone());
+
+    assert!(!set.contains(&i32::MIN));
+    assert!(!set.contains(&(range.start - 1)));
+    for i in range.clone() {
+        assert!(set.contains(&i));
+    }
+    assert!(!set.contains(&range.end));
+    assert!(!set.contains(&i32::MAX));
+}
+
+/// Use hashbrown's default hasher.
+#[test]
+fn default() {
+    check::<hashbrown::DefaultHashBuilder>();
+}
+
+/// Use std's default hasher.
+#[test]
+fn random_state() {
+    check::<std::collections::hash_map::RandomState>();
+}
+
+/// Use a constant 0 hash.
+#[test]
+fn zero() {
+    #[derive(Default)]
+    struct ZeroHasher;
+
+    impl Hasher for ZeroHasher {
+        fn finish(&self) -> u64 {
+            0
+        }
+        fn write(&mut self, _: &[u8]) {}
+    }
+
+    check::<BuildHasherDefault<ZeroHasher>>();
+}
+
+/// Use a constant maximum hash.
+#[test]
+fn max() {
+    #[derive(Default)]
+    struct MaxHasher;
+
+    impl Hasher for MaxHasher {
+        fn finish(&self) -> u64 {
+            u64::MAX
+        }
+        fn write(&mut self, _: &[u8]) {}
+    }
+
+    check::<BuildHasherDefault<MaxHasher>>();
+}