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/log/benches/value.rs

@@ -0,0 +1,27 @@
+#![cfg(feature = "kv")]
+#![feature(test)]
+
+use log::kv::Value;
+
+#[bench]
+fn u8_to_value(b: &mut test::Bencher) {
+    b.iter(|| Value::from(1u8));
+}
+
+#[bench]
+fn u8_to_value_debug(b: &mut test::Bencher) {
+    b.iter(|| Value::from_debug(&1u8));
+}
+
+#[bench]
+fn str_to_value_debug(b: &mut test::Bencher) {
+    b.iter(|| Value::from_debug(&"a string"));
+}
+
+#[bench]
+fn custom_to_value_debug(b: &mut test::Bencher) {
+    #[derive(Debug)]
+    struct A;
+
+    b.iter(|| Value::from_debug(&A));
+}