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/unicode-ident/tests/roaring/mod.rs

@@ -0,0 +1,23 @@
+#![allow(clippy::incompatible_msrv)]
+
+use roaring::RoaringBitmap;
+
+pub fn xid_start_bitmap() -> RoaringBitmap {
+    let mut bitmap = RoaringBitmap::new();
+    for ch in '\0'..=char::MAX {
+        if unicode_ident::is_xid_start(ch) {
+            bitmap.insert(ch as u32);
+        }
+    }
+    bitmap
+}
+
+pub fn xid_continue_bitmap() -> RoaringBitmap {
+    let mut bitmap = RoaringBitmap::new();
+    for ch in '\0'..=char::MAX {
+        if unicode_ident::is_xid_continue(ch) {
+            bitmap.insert(ch as u32);
+        }
+    }
+    bitmap
+}