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/proc-macro2/tests/test_fmt.rs

@@ -0,0 +1,28 @@
+#![allow(clippy::from_iter_instead_of_collect)]
+
+use proc_macro2::{Delimiter, Group, Ident, Span, TokenStream, TokenTree};
+use std::iter;
+
+#[test]
+fn test_fmt_group() {
+    let ident = Ident::new("x", Span::call_site());
+    let inner = TokenStream::from_iter(iter::once(TokenTree::Ident(ident)));
+    let parens_empty = Group::new(Delimiter::Parenthesis, TokenStream::new());
+    let parens_nonempty = Group::new(Delimiter::Parenthesis, inner.clone());
+    let brackets_empty = Group::new(Delimiter::Bracket, TokenStream::new());
+    let brackets_nonempty = Group::new(Delimiter::Bracket, inner.clone());
+    let braces_empty = Group::new(Delimiter::Brace, TokenStream::new());
+    let braces_nonempty = Group::new(Delimiter::Brace, inner.clone());
+    let none_empty = Group::new(Delimiter::None, TokenStream::new());
+    let none_nonempty = Group::new(Delimiter::None, inner);
+
+    // Matches libproc_macro.
+    assert_eq!("()", parens_empty.to_string());
+    assert_eq!("(x)", parens_nonempty.to_string());
+    assert_eq!("[]", brackets_empty.to_string());
+    assert_eq!("[x]", brackets_nonempty.to_string());
+    assert_eq!("{ }", braces_empty.to_string());
+    assert_eq!("{ x }", braces_nonempty.to_string());
+    assert_eq!("", none_empty.to_string());
+    assert_eq!("x", none_nonempty.to_string());
+}