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/common/parse.rs

@@ -0,0 +1,51 @@
+extern crate rustc_ast;
+extern crate rustc_driver;
+extern crate rustc_expand;
+extern crate rustc_parse;
+extern crate rustc_session;
+extern crate rustc_span;
+
+use rustc_ast::ast;
+use rustc_parse::lexer::StripTokens;
+use rustc_session::parse::ParseSess;
+use rustc_span::FileName;
+use std::panic;
+
+pub fn librustc_expr(input: &str) -> Option<Box<ast::Expr>> {
+    match panic::catch_unwind(|| {
+        let sess = ParseSess::new();
+        let name = FileName::Custom("test_precedence".to_string());
+        let mut parser = rustc_parse::new_parser_from_source_str(
+            &sess,
+            name,
+            input.to_string(),
+            StripTokens::ShebangAndFrontmatter,
+        )
+        .unwrap();
+        let presult = parser.parse_expr();
+        match presult {
+            Ok(expr) => Some(expr),
+            Err(diagnostic) => {
+                diagnostic.emit();
+                None
+            }
+        }
+    }) {
+        Ok(Some(e)) => Some(e),
+        Ok(None) => None,
+        Err(_) => {
+            errorf!("librustc panicked\n");
+            None
+        }
+    }
+}
+
+pub fn syn_expr(input: &str) -> Option<syn::Expr> {
+    match syn::parse_str(input) {
+        Ok(e) => Some(e),
+        Err(msg) => {
+            errorf!("syn failed to parse\n{:?}\n", msg);
+            None
+        }
+    }
+}