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/embedded-hal-async/src/lib.rs

@@ -0,0 +1,15 @@
+#![doc = include_str!("../README.md")]
+#![warn(missing_docs)]
+#![no_std]
+// disable warning for already-stabilized features.
+// Needed to pass CI, because we deny warnings.
+// We don't immediately remove them to not immediately break older nightlies.
+// When all features are stable, we'll remove them.
+#![cfg_attr(nightly, allow(stable_features, unknown_lints))]
+#![cfg_attr(nightly, feature(async_fn_in_trait, impl_trait_projections))]
+#![allow(async_fn_in_trait)]
+
+pub mod delay;
+pub mod digital;
+pub mod i2c;
+pub mod spi;