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/late-compile-pass.rs

@@ -0,0 +1,79 @@
+// Copyright 2026 The Fuchsia Authors
+//
+// Licensed under a BSD-style license <LICENSE-BSD>, 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.
+
+// These tests cause errors which are generated by a later compilation pass than
+// the other errors we generate, and so if they're compiled in the same file,
+// the compiler will never get to that pass, and so we won't get the errors.
+
+include!("../include.rs");
+
+use util::AU16;
+use zerocopy::{transmute, transmute_mut, try_transmute};
+
+// Although this is not a soundness requirement, we currently require that the
+// size of the destination type is not smaller than the size of the source type.
+const TRANSMUTE_DECREASE_SIZE: u8 = transmute!(AU16(0));
+//~[msrv, stable, nightly]^ ERROR: cannot transmute between types of different sizes, or dependently-sized types
+//~[stable, nightly]^^ ERROR: transmuting from 2-byte type to 1-byte type: `AU16` -> `u8`
+
+// `transmute!` does not support transmuting from a smaller type to a larger
+// one.
+const TRANSMUTE_INCREASE_SIZE: AU16 = transmute!(0u8);
+//~[msrv, stable, nightly]^ ERROR: cannot transmute between types of different sizes, or dependently-sized types
+//~[stable, nightly]^^ ERROR: transmuting from 1-byte type to 2-byte type: `u8` -> `AU16`
+
+// `transmute!` does not support transmuting from a smaller type to a larger
+// one.
+const TRANSMUTE_INCREASE_SIZE_ALLOW_SHRINK: AU16 = transmute!(#![allow(shrink)] 0u8);
+//~[msrv, stable, nightly]^ ERROR: cannot transmute between types of different sizes, or dependently-sized types
+//~[stable, nightly]^^ ERROR: transmuting from 1-byte type to 2-byte type: `u8` -> `Transmute<u8, AU16>`
+
+const ARRAY_OF_U8S: [u8; 2] = [0u8; 2];
+
+// `transmute_mut!` cannot, generally speaking, be used in const contexts.
+const TRANSMUTE_MUT_CONST_CONTEXT: &mut [u8; 2] = transmute_mut!(&mut ARRAY_OF_U8S);
+//~[msrv]^ ERROR: mutable references are not allowed in constants
+//~[msrv]^^ ERROR: calls in constants are limited to constant functions, tuple structs and tuple variants
+//~[msrv]^^^ ERROR: calls in constants are limited to constant functions, tuple structs and tuple variants
+//~[msrv]^^^^ ERROR: temporary value dropped while borrowed
+//~[stable]^^^^^ ERROR: cannot call non-const method `Wrap::<&mut [u8; 2], &mut [u8; 2]>::transmute_mut_inference_helper` in constants
+//~[stable]^^^^^^ ERROR: cannot call non-const method `Wrap::<&mut [u8; 2], &mut [u8; 2]>::transmute_mut` in constants
+//~[nightly]^^^^^^^ ERROR: cannot call non-const method `zerocopy::util::macro_util::Wrap::<&mut [u8; 2], &mut [u8; 2]>::transmute_mut_inference_helper` in constants
+//~[nightly]^^^^^^^^ ERROR: cannot call non-const method `zerocopy::util::macro_util::Wrap::<&mut [u8; 2], &mut [u8; 2]>::transmute_mut` in constants
+
+// Should fail because the file is 4 bytes long, not 8.
+const INCLUDE_VALUE_WRONG_SIZE: u64 = zerocopy::include_value!("../../testdata/include_value/data");
+//~[msrv, stable, nightly]^ ERROR: cannot transmute between types of different sizes, or dependently-sized types
+//~[stable, nightly]^^ ERROR: transmuting from 4-byte type to 8-byte type: `[u8; 4]` -> `u64`
+
+fn main() {
+    // Although this is not a soundness requirement, we currently require that
+    // the size of the destination type is not smaller than the size of the
+    // source type.
+    let _decrease_size: Result<u8, _> = try_transmute!(AU16(0));
+    //~[msrv, stable, nightly]^ ERROR: cannot transmute between types of different sizes, or dependently-sized types
+
+    // `try_transmute!` does not support transmuting from a smaller type to a larger
+    // one.
+    let _increase_size: Result<AU16, _> = try_transmute!(0u8);
+    //~[msrv, stable, nightly]^ ERROR: cannot transmute between types of different sizes, or dependently-sized types
+
+    fn _transmute_mut_increase_lifetime() {
+        let mut x = 0u64;
+        // It is illegal to increase the lifetime scope.
+        let _: &'static mut u64 = zerocopy::transmute_mut!(&mut x);
+        //~[msrv, stable, nightly]^ ERROR: `x` does not live long enough
+    }
+
+    fn _transmute_ref_increase_lifetime() {
+        let x = 0u64;
+        // It is illegal to increase the lifetime scope.
+        let _: &'static u64 = zerocopy::transmute_ref!(&x);
+        //~[msrv, stable, nightly]^ ERROR: `x` does not live long enough
+    }
+}