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/tests/test_size.rs

@@ -0,0 +1,54 @@
+// Assumes proc-macro2's "span-locations" feature is off.
+
+use std::mem;
+use syn::{Expr, Item, Lit, Pat, Type};
+
+#[rustversion::attr(before(2022-11-24), ignore = "requires nightly-2022-11-24 or newer")]
+#[rustversion::attr(
+    since(2022-11-24),
+    cfg_attr(not(target_pointer_width = "64"), ignore = "only applicable to 64-bit")
+)]
+#[test]
+fn test_expr_size() {
+    assert_eq!(mem::size_of::<Expr>(), 176);
+}
+
+#[rustversion::attr(before(2022-09-09), ignore = "requires nightly-2022-09-09 or newer")]
+#[rustversion::attr(
+    since(2022-09-09),
+    cfg_attr(not(target_pointer_width = "64"), ignore = "only applicable to 64-bit")
+)]
+#[test]
+fn test_item_size() {
+    assert_eq!(mem::size_of::<Item>(), 352);
+}
+
+#[rustversion::attr(before(2023-04-29), ignore = "requires nightly-2023-04-29 or newer")]
+#[rustversion::attr(
+    since(2023-04-29),
+    cfg_attr(not(target_pointer_width = "64"), ignore = "only applicable to 64-bit")
+)]
+#[test]
+fn test_type_size() {
+    assert_eq!(mem::size_of::<Type>(), 224);
+}
+
+#[rustversion::attr(before(2023-04-29), ignore = "requires nightly-2023-04-29 or newer")]
+#[rustversion::attr(
+    since(2023-04-29),
+    cfg_attr(not(target_pointer_width = "64"), ignore = "only applicable to 64-bit")
+)]
+#[test]
+fn test_pat_size() {
+    assert_eq!(mem::size_of::<Pat>(), 184);
+}
+
+#[rustversion::attr(before(2023-12-20), ignore = "requires nightly-2023-12-20 or newer")]
+#[rustversion::attr(
+    since(2023-12-20),
+    cfg_attr(not(target_pointer_width = "64"), ignore = "only applicable to 64-bit")
+)]
+#[test]
+fn test_lit_size() {
+    assert_eq!(mem::size_of::<Lit>(), 24);
+}