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/zerocopy/tests/ui/ptr-is-invariant-over-v.rs

@@ -0,0 +1,35 @@
+// Copyright 2025 The Fuchsia Authors
+//
+// Licensed under the 2-Clause BSD License <LICENSE-BSD or
+// https://opensource.org/license/bsd-2-clause>, Apache License, Version 2.0
+// <LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0>, or the MIT
+// license <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your option.
+// This file may not be copied, modified, or distributed except according to
+// those terms.
+
+include!("../include.rs");
+
+use zerocopy::pointer::{
+    invariant::{Aligned, Exclusive, Shared, Valid},
+    Ptr,
+};
+
+fn _when_exclusive<'big: 'small, 'small>(
+    big: Ptr<'small, &'big u32, (Exclusive, Aligned, Valid)>,
+    mut _small: Ptr<'small, &'small u32, (Exclusive, Aligned, Valid)>,
+) {
+    _small = big;
+    //~[msrv]^ ERROR: lifetime mismatch
+    //~[stable, nightly]^^ ERROR: lifetime may not live long enough
+}
+
+fn _when_shared<'big: 'small, 'small>(
+    big: Ptr<'small, &'big u32, (Shared, Aligned, Valid)>,
+    mut _small: Ptr<'small, &'small u32, (Shared, Aligned, Valid)>,
+) {
+    _small = big;
+    //~[msrv]^ ERROR: lifetime mismatch
+    //~[stable, nightly]^^ ERROR: lifetime may not live long enough
+}
+
+fn main() {}