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/syn-1.0.109/tests/test_size.rs

@@ -0,0 +1,29 @@
+#![cfg(target_pointer_width = "64")]
+
+use std::mem;
+use syn::{Expr, Item, Lit, Pat, Type};
+
+#[test]
+fn test_expr_size() {
+    assert_eq!(mem::size_of::<Expr>(), 272);
+}
+
+#[test]
+fn test_item_size() {
+    assert_eq!(mem::size_of::<Item>(), 320);
+}
+
+#[test]
+fn test_type_size() {
+    assert_eq!(mem::size_of::<Type>(), 288);
+}
+
+#[test]
+fn test_pat_size() {
+    assert_eq!(mem::size_of::<Pat>(), 144);
+}
+
+#[test]
+fn test_lit_size() {
+    assert_eq!(mem::size_of::<Lit>(), 32);
+}