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/indexmap/tests/tests.rs

@@ -0,0 +1,28 @@
+use indexmap::{indexmap, indexset};
+
+#[test]
+fn test_sort() {
+    let m = indexmap! {
+        1 => 2,
+        7 => 1,
+        2 => 2,
+        3 => 3,
+    };
+
+    itertools::assert_equal(
+        m.sorted_by(|_k1, v1, _k2, v2| v1.cmp(v2)),
+        vec![(7, 1), (1, 2), (2, 2), (3, 3)],
+    );
+}
+
+#[test]
+fn test_sort_set() {
+    let s = indexset! {
+        1,
+        7,
+        2,
+        3,
+    };
+
+    itertools::assert_equal(s.sorted_by(|v1, v2| v1.cmp(v2)), vec![1, 2, 3, 7]);
+}