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_meta.rs

@@ -0,0 +1,180 @@
+#![allow(
+    clippy::elidable_lifetime_names,
+    clippy::needless_lifetimes,
+    clippy::shadow_unrelated,
+    clippy::too_many_lines,
+    clippy::uninlined_format_args
+)]
+
+#[macro_use]
+mod snapshot;
+
+mod debug;
+
+use quote::quote;
+use syn::parse::{ParseStream, Parser as _, Result};
+use syn::{Meta, MetaList, MetaNameValue, Token};
+
+#[test]
+fn test_parse_meta_item_word() {
+    let input = "hello";
+
+    snapshot!(input as Meta, @r#"
+    Meta::Path {
+        segments: [
+            PathSegment {
+                ident: "hello",
+            },
+        ],
+    }
+    "#);
+}
+
+#[test]
+fn test_parse_meta_name_value() {
+    let input = "foo = 5";
+    let (inner, meta) = (input, input);
+
+    snapshot!(inner as MetaNameValue, @r#"
+    MetaNameValue {
+        path: Path {
+            segments: [
+                PathSegment {
+                    ident: "foo",
+                },
+            ],
+        },
+        value: Expr::Lit {
+            lit: 5,
+        },
+    }
+    "#);
+
+    snapshot!(meta as Meta, @r#"
+    Meta::NameValue {
+        path: Path {
+            segments: [
+                PathSegment {
+                    ident: "foo",
+                },
+            ],
+        },
+        value: Expr::Lit {
+            lit: 5,
+        },
+    }
+    "#);
+
+    assert_eq!(meta, Meta::NameValue(inner));
+}
+
+#[test]
+fn test_parse_meta_item_list_lit() {
+    let input = "foo(5)";
+    let (inner, meta) = (input, input);
+
+    snapshot!(inner as MetaList, @r#"
+    MetaList {
+        path: Path {
+            segments: [
+                PathSegment {
+                    ident: "foo",
+                },
+            ],
+        },
+        delimiter: MacroDelimiter::Paren,
+        tokens: TokenStream(`5`),
+    }
+    "#);
+
+    snapshot!(meta as Meta, @r#"
+    Meta::List {
+        path: Path {
+            segments: [
+                PathSegment {
+                    ident: "foo",
+                },
+            ],
+        },
+        delimiter: MacroDelimiter::Paren,
+        tokens: TokenStream(`5`),
+    }
+    "#);
+
+    assert_eq!(meta, Meta::List(inner));
+}
+
+#[test]
+fn test_parse_meta_item_multiple() {
+    let input = "foo(word, name = 5, list(name2 = 6), word2)";
+    let (inner, meta) = (input, input);
+
+    snapshot!(inner as MetaList, @r#"
+    MetaList {
+        path: Path {
+            segments: [
+                PathSegment {
+                    ident: "foo",
+                },
+            ],
+        },
+        delimiter: MacroDelimiter::Paren,
+        tokens: TokenStream(`word , name = 5 , list (name2 = 6) , word2`),
+    }
+    "#);
+
+    snapshot!(meta as Meta, @r#"
+    Meta::List {
+        path: Path {
+            segments: [
+                PathSegment {
+                    ident: "foo",
+                },
+            ],
+        },
+        delimiter: MacroDelimiter::Paren,
+        tokens: TokenStream(`word , name = 5 , list (name2 = 6) , word2`),
+    }
+    "#);
+
+    assert_eq!(meta, Meta::List(inner));
+}
+
+#[test]
+fn test_parse_path() {
+    let input = "::serde::Serialize";
+    snapshot!(input as Meta, @r#"
+    Meta::Path {
+        leading_colon: Some,
+        segments: [
+            PathSegment {
+                ident: "serde",
+            },
+            Token![::],
+            PathSegment {
+                ident: "Serialize",
+            },
+        ],
+    }
+    "#);
+}
+
+#[test]
+fn test_fat_arrow_after_meta() {
+    fn parse(input: ParseStream) -> Result<()> {
+        while !input.is_empty() {
+            let _: Meta = input.parse()?;
+            let _: Token![=>] = input.parse()?;
+            let brace;
+            syn::braced!(brace in input);
+        }
+        Ok(())
+    }
+
+    let input = quote! {
+        target_os = "linux" => {}
+        windows => {}
+    };
+
+    parse.parse2(input).unwrap();
+}