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/crunchy/src/lib.rs

@@ -0,0 +1,37 @@
+//! The crunchy unroller - deterministically unroll constant loops. For number "crunching".
+//!
+//! The Rust optimizer will unroll constant loops that don't use the loop variable, like this:
+//!
+//! ```ignore
+//! for _ in 0..100 {
+//!     println!("Hello!");
+//! }
+//! ```
+//!
+//! However, using the loop variable will cause it to never unroll the loop. This is unfortunate because it means that you can't
+//! constant-fold the loop variable, and if you end up stomping on the registers it will have to do a load for each iteration.
+//! This crate ensures that your code is unrolled and const-folded. It only works on literals,
+//! unfortunately, but there's a work-around:
+//!
+//! ```ignore
+//! debug_assert_eq!(MY_CONSTANT, 100);
+//! unroll! {
+//!     for i in 0..100 {
+//!         println!("Iteration {}", i);
+//!     }
+//! }
+//! ```
+//! This means that your tests will catch if you redefine the constant.
+//!
+//! To default maximum number of loops to unroll is `64`, but that can be easily increased using the cargo features:
+//!
+//! * `limit_128`
+//! * `limit_256`
+//! * `limit_512`
+//! * `limit_1024`
+//! * `limit_2048`
+
+#![cfg_attr(not(feature = "std"), no_std)]
+
+include!(concat!(env!("OUT_DIR"), env!("CRUNCHY_LIB_SUFFIX")));
+