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_mut.rs

@@ -0,0 +1,89 @@
+// 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_mut;
+
+fn main() {}
+
+// `transmute_mut!` does not support transmuting into a non-reference
+// destination type.
+const DST_NOT_A_REFERENCE: usize = transmute_mut!(&mut 0u8);
+//~[msrv, stable, nightly]^ ERROR: mismatched types
+//~[msrv, stable, nightly]^^ ERROR: mismatched types
+
+#[derive(zerocopy::FromBytes, zerocopy::IntoBytes, zerocopy::Immutable)]
+#[repr(C)]
+struct SrcA;
+
+#[derive(zerocopy::IntoBytes, zerocopy::Immutable)]
+#[repr(C)]
+struct DstA;
+
+// `transmute_mut` requires that the destination type implements `FromBytes`
+const DST_NOT_FROM_BYTES: &mut DstA = transmute_mut!(&mut SrcA);
+//~[msrv, stable, nightly]^ ERROR: the trait bound `DstA: FromBytes` is not satisfied
+
+#[derive(zerocopy::FromBytes, zerocopy::IntoBytes, zerocopy::Immutable)]
+#[repr(C)]
+struct SrcB;
+
+#[derive(zerocopy::FromBytes, zerocopy::Immutable)]
+#[repr(C)]
+struct DstB;
+
+// `transmute_mut` requires that the destination type implements `IntoBytes`
+const DST_NOT_AS_BYTES: &mut DstB = transmute_mut!(&mut SrcB);
+//~[msrv, stable, nightly]^ ERROR: the trait bound `DstB: IntoBytes` is not satisfied
+
+// `transmute_mut!` does not support transmuting between non-reference source
+// and destination types.
+const SRC_DST_NOT_REFERENCES: &mut usize = transmute_mut!(0usize);
+//~[msrv, stable, nightly]^ ERROR: mismatched types
+
+fn ref_src_immutable() {
+    // `transmute_mut!` requires that its source type be a mutable reference.
+    let _: &mut u8 = transmute_mut!(&0u8);
+    //~[msrv, stable, nightly]^ ERROR: mismatched types
+}
+
+// `transmute_mut!` does not support transmuting from a non-reference source
+// type.
+const SRC_NOT_A_REFERENCE: &mut u8 = transmute_mut!(0usize);
+//~[msrv, stable, nightly]^ ERROR: mismatched types
+
+#[derive(zerocopy::IntoBytes, zerocopy::Immutable)]
+#[repr(C)]
+struct SrcC;
+
+#[derive(zerocopy::FromBytes, zerocopy::IntoBytes, zerocopy::Immutable)]
+#[repr(C)]
+struct DstC;
+
+// `transmute_mut` requires that the source type implements `FromBytes`
+const SRC_NOT_FROM_BYTES: &mut DstC = transmute_mut!(&mut SrcC);
+//~[msrv, stable, nightly]^ ERROR: the trait bound `SrcC: FromBytes` is not satisfied
+
+#[derive(zerocopy::FromBytes, zerocopy::Immutable)]
+#[repr(C)]
+struct SrcD;
+
+#[derive(zerocopy::FromBytes, zerocopy::IntoBytes, zerocopy::Immutable)]
+#[repr(C)]
+struct DstD;
+
+// `transmute_mut` requires that the source type implements `IntoBytes`
+const SRC_NOT_AS_BYTES: &mut DstD = transmute_mut!(&mut SrcD);
+//~[msrv, stable, nightly]^ ERROR: the trait bound `SrcD: IntoBytes` is not satisfied
+
+// `transmute_mut!` does not support transmuting from an unsized source type to
+// a sized destination type.
+const SRC_UNSIZED: &mut [u8; 1] = transmute_mut!(&mut [0u8][..]);
+//~[msrv, stable]^ ERROR: the method `transmute_mut` exists for struct `Wrap<&mut [u8], &mut [u8; 1]>`, but its trait bounds were not satisfied
+//~[nightly]^^ ERROR: transmute_mut` exists for struct `zerocopy::util::macro_util::Wrap<&mut [u8], &mut [u8; 1]>`, but its trait bounds were not satisfied