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/embassy-hal-internal/src/lib.rs

@@ -0,0 +1,17 @@
+#![no_std]
+#![allow(clippy::new_without_default)]
+#![doc = include_str!("../README.md")]
+#![warn(missing_docs)]
+
+// This mod MUST go first, so that the others see its macros.
+pub(crate) mod fmt;
+
+pub mod atomic_ring_buffer;
+pub mod drop;
+mod macros;
+mod peripheral;
+pub mod ratio;
+pub use peripheral::{Peripheral, PeripheralRef};
+
+#[cfg(feature = "cortex-m")]
+pub mod interrupt;