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_ident.rs

@@ -0,0 +1,87 @@
+use proc_macro2::{Ident, Span, TokenStream};
+use std::str::FromStr;
+use syn::Result;
+
+#[track_caller]
+fn parse(s: &str) -> Result<Ident> {
+    syn::parse2(TokenStream::from_str(s).unwrap())
+}
+
+#[track_caller]
+fn new(s: &str) -> Ident {
+    Ident::new(s, Span::call_site())
+}
+
+#[test]
+fn ident_parse() {
+    parse("String").unwrap();
+}
+
+#[test]
+fn ident_parse_keyword() {
+    parse("abstract").unwrap_err();
+}
+
+#[test]
+fn ident_parse_empty() {
+    parse("").unwrap_err();
+}
+
+#[test]
+fn ident_parse_lifetime() {
+    parse("'static").unwrap_err();
+}
+
+#[test]
+fn ident_parse_underscore() {
+    parse("_").unwrap_err();
+}
+
+#[test]
+fn ident_parse_number() {
+    parse("255").unwrap_err();
+}
+
+#[test]
+fn ident_parse_invalid() {
+    parse("a#").unwrap_err();
+}
+
+#[test]
+fn ident_new() {
+    new("String");
+}
+
+#[test]
+fn ident_new_keyword() {
+    new("abstract");
+}
+
+#[test]
+#[should_panic(expected = "use Option<Ident>")]
+fn ident_new_empty() {
+    new("");
+}
+
+#[test]
+#[should_panic(expected = "not a valid Ident")]
+fn ident_new_lifetime() {
+    new("'static");
+}
+
+#[test]
+fn ident_new_underscore() {
+    new("_");
+}
+
+#[test]
+#[should_panic(expected = "use Literal instead")]
+fn ident_new_number() {
+    new("255");
+}
+
+#[test]
+#[should_panic(expected = "\"a#\" is not a valid Ident")]
+fn ident_new_invalid() {
+    new("a#");
+}