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_shebang.rs

@@ -0,0 +1,73 @@
+#![allow(
+    clippy::elidable_lifetime_names,
+    clippy::needless_lifetimes,
+    clippy::uninlined_format_args
+)]
+
+#[macro_use]
+mod snapshot;
+
+mod debug;
+
+#[test]
+fn test_basic() {
+    let content = "#!/usr/bin/env rustx\nfn main() {}";
+    let file = syn::parse_file(content).unwrap();
+    snapshot!(file, @r##"
+    File {
+        shebang: Some("#!/usr/bin/env rustx"),
+        items: [
+            Item::Fn {
+                vis: Visibility::Inherited,
+                sig: Signature {
+                    ident: "main",
+                    generics: Generics,
+                    output: ReturnType::Default,
+                },
+                block: Block {
+                    stmts: [],
+                },
+            },
+        ],
+    }
+    "##);
+}
+
+#[test]
+fn test_comment() {
+    let content = "#!//am/i/a/comment\n[allow(dead_code)] fn main() {}";
+    let file = syn::parse_file(content).unwrap();
+    snapshot!(file, @r#"
+    File {
+        attrs: [
+            Attribute {
+                style: AttrStyle::Inner,
+                meta: Meta::List {
+                    path: Path {
+                        segments: [
+                            PathSegment {
+                                ident: "allow",
+                            },
+                        ],
+                    },
+                    delimiter: MacroDelimiter::Paren,
+                    tokens: TokenStream(`dead_code`),
+                },
+            },
+        ],
+        items: [
+            Item::Fn {
+                vis: Visibility::Inherited,
+                sig: Signature {
+                    ident: "main",
+                    generics: Generics,
+                    output: ReturnType::Default,
+                },
+                block: Block {
+                    stmts: [],
+                },
+            },
+        ],
+    }
+    "#);
+}