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_stmt.rs

@@ -0,0 +1,93 @@
+#![allow(clippy::assertions_on_result_states, clippy::non_ascii_literal)]
+
+#[macro_use]
+mod macros;
+
+use proc_macro2::{Delimiter, Group, Ident, Span, TokenStream, TokenTree};
+use quote::quote;
+use std::iter::FromIterator;
+use syn::Stmt;
+
+#[test]
+fn test_raw_operator() {
+    let stmt = syn::parse_str::<Stmt>("let _ = &raw const x;").unwrap();
+
+    snapshot!(stmt, @r###"
+    Local(Local {
+        pat: Pat::Wild,
+        init: Some(Verbatim(`& raw const x`)),
+    })
+    "###);
+}
+
+#[test]
+fn test_raw_variable() {
+    let stmt = syn::parse_str::<Stmt>("let _ = &raw;").unwrap();
+
+    snapshot!(stmt, @r###"
+    Local(Local {
+        pat: Pat::Wild,
+        init: Some(Expr::Reference {
+            expr: Expr::Path {
+                path: Path {
+                    segments: [
+                        PathSegment {
+                            ident: "raw",
+                            arguments: None,
+                        },
+                    ],
+                },
+            },
+        }),
+    })
+    "###);
+}
+
+#[test]
+fn test_raw_invalid() {
+    assert!(syn::parse_str::<Stmt>("let _ = &raw x;").is_err());
+}
+
+#[test]
+fn test_none_group() {
+    // <Ø async fn f() {} Ø>
+    let tokens = TokenStream::from_iter(vec![TokenTree::Group(Group::new(
+        Delimiter::None,
+        TokenStream::from_iter(vec![
+            TokenTree::Ident(Ident::new("async", Span::call_site())),
+            TokenTree::Ident(Ident::new("fn", Span::call_site())),
+            TokenTree::Ident(Ident::new("f", Span::call_site())),
+            TokenTree::Group(Group::new(Delimiter::Parenthesis, TokenStream::new())),
+            TokenTree::Group(Group::new(Delimiter::Brace, TokenStream::new())),
+        ]),
+    ))]);
+
+    snapshot!(tokens as Stmt, @r###"
+    Item(Item::Fn {
+        vis: Inherited,
+        sig: Signature {
+            asyncness: Some,
+            ident: "f",
+            generics: Generics,
+            output: Default,
+        },
+        block: Block,
+    })
+    "###);
+}
+
+#[test]
+fn test_let_dot_dot() {
+    let tokens = quote! {
+        let .. = 10;
+    };
+
+    snapshot!(tokens as Stmt, @r###"
+    Local(Local {
+        pat: Pat::Rest,
+        init: Some(Expr::Lit {
+            lit: 10,
+        }),
+    })
+    "###);
+}