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/README.md

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+# Crunchy
+
+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:
+
+```rust
+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:
+
+```rust
+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 `128`, but that can be easily decreased or increased using the cargo features:
+
+* `limit_64`
+* `limit_128`
+* `limit_256`
+* `limit_512`
+* `limit_1024`
+* `limit_2048`