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/indexmap/src/sval.rs

@@ -0,0 +1,36 @@
+#![cfg_attr(docsrs, doc(cfg(feature = "sval")))]
+
+use crate::{IndexMap, IndexSet};
+use sval::{Stream, Value};
+
+impl<K: Value, V: Value, S> Value for IndexMap<K, V, S> {
+    fn stream<'sval, ST: Stream<'sval> + ?Sized>(&'sval self, stream: &mut ST) -> sval::Result {
+        stream.map_begin(Some(self.len()))?;
+
+        for (k, v) in self {
+            stream.map_key_begin()?;
+            stream.value(k)?;
+            stream.map_key_end()?;
+
+            stream.map_value_begin()?;
+            stream.value(v)?;
+            stream.map_value_end()?;
+        }
+
+        stream.map_end()
+    }
+}
+
+impl<K: Value, S> Value for IndexSet<K, S> {
+    fn stream<'sval, ST: Stream<'sval> + ?Sized>(&'sval self, stream: &mut ST) -> sval::Result {
+        stream.seq_begin(Some(self.len()))?;
+
+        for value in self {
+            stream.seq_value_begin()?;
+            stream.value(value)?;
+            stream.seq_value_end()?;
+        }
+
+        stream.seq_end()
+    }
+}