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/diagnostic-not-implemented.rs

@@ -0,0 +1,83 @@
+// Copyright 2022 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 util::NotZerocopy;
+use zerocopy::{FromBytes, FromZeros, Immutable, IntoBytes, KnownLayout, TryFromBytes, Unaligned};
+
+fn main() {
+    // We expect the proper diagnostic to be emitted on Rust 1.78.0 and later.
+    takes_from_bytes::<NotZerocopy>();
+    //~[msrv, stable, nightly]^ ERROR: the trait bound `NotZerocopy: FromBytes` is not satisfied
+    takes_from_zeros::<NotZerocopy>();
+    //~[msrv, stable, nightly]^ ERROR: the trait bound `NotZerocopy: FromZeros` is not satisfied
+    takes_immutable::<NotZerocopy>();
+    //~[msrv, stable, nightly]^ ERROR: the trait bound `NotZerocopy: Immutable` is not satisfied
+    takes_into_bytes::<NotZerocopy>();
+    //~[msrv, stable, nightly]^ ERROR: the trait bound `NotZerocopy: IntoBytes` is not satisfied
+    takes_known_layout::<NotZerocopy>();
+    //~[msrv, stable, nightly]^ ERROR: the trait bound `NotZerocopy: KnownLayout` is not satisfied
+    takes_try_from_bytes::<NotZerocopy>();
+    //~[msrv, stable, nightly]^ ERROR: the trait bound `NotZerocopy: TryFromBytes` is not satisfied
+    takes_unaligned::<NotZerocopy>();
+    //~[msrv, stable, nightly]^ ERROR: the trait bound `NotZerocopy: zerocopy::Unaligned` is not satisfied
+
+    // This is adapted from #1296, which includes the following text:
+    //
+    //   The compiler errors when a type is missing Immutable are somewhat
+    //   misleading, although I'm not sure there's much zerocopy can do about
+    //   this. An example where the compiler recommends adding a reference
+    //   rather than implementing Immutable (some were even more confusing than
+    //   this):
+    //
+    //   error[E0277]: the trait bound `virtio::wl::CtrlVfdNewDmabuf: zerocopy::Immutable` is not satisfied
+    //      --> devices/src/virtio/wl.rs:317:20
+    //       |
+    //   317 |         .write_obj(ctrl_vfd_new_dmabuf)
+    //       |          --------- ^^^^^^^^^^^^^^^^^^^ the trait `zerocopy::Immutable` is not implemented for `virtio::wl::CtrlVfdNewDmabuf`
+    //       |          |
+    //       |          required by a bound introduced by this call
+    //       |
+    //   note: required by a bound in `virtio::descriptor_utils::Writer::write_obj`
+    //      --> devices/src/virtio/descriptor_utils.rs:536:25
+    //       |
+    //   536 |     pub fn write_obj<T: Immutable + IntoBytes>(&mut self, val: T) -> io::Result<()> {
+    //       |                         ^^^^^^^^^ required by this bound in `Writer::write_obj`
+    //   help: consider borrowing here
+    //       |
+    //   317 |         .write_obj(&ctrl_vfd_new_dmabuf)
+    //       |                    +
+    //   317 |         .write_obj(&mut ctrl_vfd_new_dmabuf)
+    //       |                    ++++
+    //
+    //   Taking the compiler's suggestion results in a different error with a
+    //   recommendation to remove the reference (back to the original code).
+    //
+    // As of this writing, the described problem is still happening thanks to
+    // https://github.com/rust-lang/rust/issues/130563. We include this test so
+    // that we can capture the current behavior, but we will update it once that
+    // Rust issue is fixed.
+    Foo.write_obj(NotZerocopy(()));
+    //~[msrv, stable, nightly]^ ERROR: the trait bound `NotZerocopy: Immutable` is not satisfied
+    //~[msrv, stable, nightly]^^ ERROR: the trait bound `NotZerocopy: IntoBytes` is not satisfied
+
+    struct Foo;
+
+    impl Foo {
+        fn write_obj<T: Immutable + IntoBytes>(&mut self, _val: T) {}
+    }
+}
+
+fn takes_from_bytes<T: FromBytes>() {}
+fn takes_from_zeros<T: FromZeros>() {}
+fn takes_immutable<T: Immutable>() {}
+fn takes_into_bytes<T: IntoBytes>() {}
+fn takes_known_layout<T: KnownLayout>() {}
+fn takes_try_from_bytes<T: TryFromBytes>() {}
+fn takes_unaligned<T: Unaligned>() {}