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_grouping.rs

@@ -0,0 +1,59 @@
+#![allow(
+    clippy::elidable_lifetime_names,
+    clippy::needless_lifetimes,
+    clippy::uninlined_format_args
+)]
+
+#[macro_use]
+mod snapshot;
+
+mod debug;
+
+use proc_macro2::{Delimiter, Group, Literal, Punct, Spacing, TokenStream, TokenTree};
+use syn::Expr;
+
+#[test]
+fn test_grouping() {
+    let tokens: TokenStream = TokenStream::from_iter([
+        TokenTree::Literal(Literal::i32_suffixed(1)),
+        TokenTree::Punct(Punct::new('+', Spacing::Alone)),
+        TokenTree::Group(Group::new(
+            Delimiter::None,
+            TokenStream::from_iter([
+                TokenTree::Literal(Literal::i32_suffixed(2)),
+                TokenTree::Punct(Punct::new('+', Spacing::Alone)),
+                TokenTree::Literal(Literal::i32_suffixed(3)),
+            ]),
+        )),
+        TokenTree::Punct(Punct::new('*', Spacing::Alone)),
+        TokenTree::Literal(Literal::i32_suffixed(4)),
+    ]);
+
+    assert_eq!(tokens.to_string(), "1i32 + 2i32 + 3i32 * 4i32");
+
+    snapshot!(tokens as Expr, @r#"
+    Expr::Binary {
+        left: Expr::Lit {
+            lit: 1i32,
+        },
+        op: BinOp::Add,
+        right: Expr::Binary {
+            left: Expr::Group {
+                expr: Expr::Binary {
+                    left: Expr::Lit {
+                        lit: 2i32,
+                    },
+                    op: BinOp::Add,
+                    right: Expr::Lit {
+                        lit: 3i32,
+                    },
+                },
+            },
+            op: BinOp::Mul,
+            right: Expr::Lit {
+                lit: 4i32,
+            },
+        },
+    }
+    "#);
+}