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/darling/tests/hash_map.rs

@@ -0,0 +1,42 @@
+use std::collections::HashMap;
+
+use darling::FromMeta;
+use syn::{parse_quote, Attribute, Path};
+
+#[derive(Debug, FromMeta, PartialEq, Eq)]
+struct MapValue {
+    name: String,
+    #[darling(default)]
+    option: bool,
+}
+
+#[test]
+fn parse_map() {
+    let attr: Attribute = parse_quote! {
+        #[foo(first(name = "Hello", option), the::second(name = "Second"))]
+    };
+
+    let meta = attr.meta;
+    let map: HashMap<Path, MapValue> = FromMeta::from_meta(&meta).unwrap();
+
+    let comparison: HashMap<Path, MapValue> = vec![
+        (
+            parse_quote!(first),
+            MapValue {
+                name: "Hello".into(),
+                option: true,
+            },
+        ),
+        (
+            parse_quote!(the::second),
+            MapValue {
+                name: "Second".into(),
+                option: false,
+            },
+        ),
+    ]
+    .into_iter()
+    .collect();
+
+    assert_eq!(comparison, map);
+}