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/src/verbatim.rs

@@ -0,0 +1,33 @@
+use crate::ext::TokenStreamExt as _;
+use crate::parse::ParseStream;
+use core::cmp::Ordering;
+use proc_macro2::{Delimiter, TokenStream};
+
+pub(crate) fn between<'a>(begin: ParseStream<'a>, end: ParseStream<'a>) -> TokenStream {
+    let end = end.cursor();
+    let mut cursor = begin.cursor();
+    assert!(crate::buffer::same_buffer(end, cursor));
+
+    let mut tokens = TokenStream::new();
+    while cursor != end {
+        let (tt, next) = cursor.token_tree().unwrap();
+
+        if crate::buffer::cmp_assuming_same_buffer(end, next) == Ordering::Less {
+            // A syntax node can cross the boundary of a None-delimited group
+            // due to such groups being transparent to the parser in most cases.
+            // Any time this occurs the group is known to be semantically
+            // irrelevant. https://github.com/dtolnay/syn/issues/1235
+            if let Some((inside, _span, after)) = cursor.group(Delimiter::None) {
+                assert!(next == after);
+                cursor = inside;
+                continue;
+            } else {
+                panic!("verbatim end must not be inside a delimited group");
+            }
+        }
+
+        tokens.append(tt);
+        cursor = next;
+    }
+    tokens
+}