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/micromath/src/statistics.rs

@@ -0,0 +1,23 @@
+//! Statistical analysis support.
+//!
+//! The `statistics` Cargo feature must be enabled to use this functionality.
+//!
+//! The following traits are available and impl'd for slices and iterators of
+//! `f32` (and can be impl'd for other types):
+//!
+//! - [Mean] - compute arithmetic mean with the `mean()` method.
+//! - [StdDev] - compute standard deviation with the `stddev()` method
+//! - [Trim] - cull outliers from a sample slice with the `trim()` method.
+//! - [Variance] - compute variance with the `variance() method.
+//!
+//! [Mean]: https://docs.rs/micromath/latest/micromath/statistics/trait.Mean.html
+//! [StdDev]: https://docs.rs/micromath/latest/micromath/statistics/trait.StdDev.html
+//! [Trim]: https://docs.rs/micromath/latest/micromath/statistics/trim/trait.Trim.html
+//! [Variance]: https://docs.rs/micromath/latest/micromath/statistics/trait.Variance.html
+
+mod mean;
+mod stddev;
+pub mod trim;
+mod variance;
+
+pub use self::{mean::Mean, stddev::StdDev, trim::Trim, variance::Variance};