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/set.rs

@@ -0,0 +1,34 @@
+#![cfg(not(miri))] // FIXME: takes too long
+
+use hashbrown::HashSet;
+use rand::{distr::Alphanumeric, rngs::SmallRng, Rng, SeedableRng};
+use std::iter;
+
+#[test]
+fn test_hashset_insert_remove() {
+    let mut m: HashSet<Vec<char>> = HashSet::new();
+    let seed = u64::from_le_bytes(*b"testseed");
+
+    let rng = &mut SmallRng::seed_from_u64(seed);
+    let tx: Vec<Vec<char>> = iter::repeat_with(|| {
+        rng.sample_iter(&Alphanumeric)
+            .take(32)
+            .map(char::from)
+            .collect()
+    })
+    .take(4096)
+    .collect();
+
+    // more readable with explicit `true` / `false`
+    #[allow(clippy::bool_assert_comparison)]
+    for _ in 0..32 {
+        for x in &tx {
+            assert_eq!(m.contains(x), false);
+            assert_eq!(m.insert(x.clone()), true);
+        }
+        for (i, x) in tx.iter().enumerate() {
+            println!("removing {i} {x:?}");
+            assert_eq!(m.remove(x), true);
+        }
+    }
+}