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/transmute_ref.rs

@@ -0,0 +1,86 @@
+// Copyright 2023 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.
+
+include!("../include.rs");
+
+use zerocopy::transmute_ref;
+
+use crate::util::AU16;
+
+fn main() {}
+
+fn ref_dst_mutable() {
+    // `transmute_ref!` requires that its destination type be an immutable
+    // reference.
+    let _: &mut u8 = transmute_ref!(&0u8);
+    //~[msrv, stable, nightly]^ ERROR: mismatched types
+    //~[msrv, stable, nightly]^^ ERROR: mismatched types
+    //~[msrv, stable, nightly]^^^ ERROR: mismatched types
+    //~[msrv, stable, nightly]^^^^ ERROR: mismatched types
+}
+
+// `transmute_ref!` does not support transmuting into a non-reference
+// destination type.
+const DST_NOT_A_REFERENCE: usize = transmute_ref!(&0u8);
+//~[msrv, stable, nightly]^ ERROR: mismatched types
+//~[msrv, stable, nightly]^^ ERROR: mismatched types
+//~[msrv, stable, nightly]^^^ ERROR: mismatched types
+//~[msrv, stable, nightly]^^^^ ERROR: mismatched types
+
+#[derive(zerocopy::Immutable)]
+#[repr(transparent)]
+struct DstA(AU16);
+
+// `transmute_ref` requires that the destination type implements `FromBytes`
+const DST_NOT_FROM_BYTES: &DstA = transmute_ref!(&AU16(0));
+//~[msrv, stable, nightly]^ ERROR: the trait bound `DstA: FromBytes` is not satisfied
+
+#[derive(zerocopy::FromBytes)]
+#[repr(transparent)]
+struct DstB(AU16);
+
+// `transmute_ref` requires that the destination type implements `Immutable`
+const DST_NOT_IMMUTABLE: &DstB = transmute_ref!(&AU16(0));
+//~[msrv, stable, nightly]^ ERROR: the trait bound `DstB: Immutable` is not satisfied
+
+// `transmute_ref!` does not support transmuting between non-reference source
+// and destination types.
+const SRC_DST_NOT_REFERENCES: usize = transmute_ref!(0usize);
+//~[msrv, stable, nightly]^ ERROR: mismatched types
+//~[msrv, stable, nightly]^^ ERROR: mismatched types
+//~[msrv, stable, nightly]^^^ ERROR: mismatched types
+//~[msrv, stable, nightly]^^^^ ERROR: mismatched types
+//~[msrv, stable, nightly]^^^^^ ERROR: mismatched types
+
+// `transmute_ref!` does not support transmuting from a non-reference source
+// type.
+const SRC_NOT_A_REFERENCE: &u8 = transmute_ref!(0usize);
+//~[msrv, stable, nightly]^ ERROR: mismatched types
+
+#[derive(zerocopy::Immutable)]
+#[repr(transparent)]
+struct SrcA(AU16);
+
+// `transmute_ref` requires that the source type implements `IntoBytes`
+const SRC_NOT_AS_BYTES: &AU16 = transmute_ref!(&SrcA(AU16(0)));
+//~[msrv, stable, nightly]^ ERROR: the trait bound `SrcA: IntoBytes` is not satisfied
+//~[msrv, stable, nightly]^^ ERROR: the trait bound `SrcA: IntoBytes` is not satisfied
+
+#[derive(zerocopy::IntoBytes)]
+#[repr(transparent)]
+struct SrcB(AU16);
+
+// `transmute_ref` requires that the source type implements `Immutable`
+const SRC_NOT_IMMUTABLE: &AU16 = transmute_ref!(&SrcB(AU16(0)));
+//~[msrv, stable, nightly]^ ERROR: the trait bound `SrcB: Immutable` is not satisfied
+//~[msrv, stable, nightly]^^ ERROR: the trait bound `SrcB: Immutable` is not satisfied
+
+// `transmute_ref!` does not support transmuting from an unsized source type.
+const SRC_UNSIZED: &[u8; 1] = transmute_ref!(&[0u8][..]);
+//~[msrv, stable]^ ERROR: the method `transmute_ref` exists for struct `Wrap<&[u8], &[u8; 1]>`, but its trait bounds were not satisfied
+//~[nightly]^^ ERROR: the method `transmute_ref` exists for struct `zerocopy::util::macro_util::Wrap<&[u8], &[u8; 1]>`, but its trait bounds were not satisfied