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/winnow/examples/json/json.rs

@@ -0,0 +1,11 @@
+use std::collections::HashMap;
+
+#[derive(Debug, PartialEq, Clone)]
+pub(crate) enum JsonValue {
+    Null,
+    Boolean(bool),
+    Str(String),
+    Num(f64),
+    Array(Vec<JsonValue>),
+    Object(HashMap<String, JsonValue>),
+}