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/az/README.md

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+<!-- Copyright © 2019–2021 Trevor Spiteri -->
+
+<!-- Copying and distribution of this file, with or without
+modification, are permitted in any medium without royalty provided the
+copyright notice and this notice are preserved. This file is offered
+as-is, without any warranty. -->
+
+# Numeric casts
+
+This crate provides casts and checked casts.
+
+# What’s new
+
+### Version 1.2.1 news (2022-07-25)
+
+  * Fix build issue using rustc 1.64.0-nightly under Windows.
+
+### Version 1.2.0 news (2021-11-24)
+
+  * The following traits were added, which can be used for constraints in the
+    opposite direction to the other cast traits.
+      * [CastFrom][cf-1-2]
+      * [CheckedCastFrom][ccf-1-2]
+      * [SaturatingCastFrom][scf-1-2]
+      * [WrappingCastFrom][wcf-1-2]
+      * [OverflowingCastFrom][ocf-1-2]
+      * [UnwrappedCastFrom][ucf-1-2]
+
+[ccf-1-2]: https://docs.rs/az/~1.2/az/trait.CheckedCastFrom.html
+[cf-1-2]: https://docs.rs/az/~1.2/az/trait.CastFrom.html
+[ocf-1-2]: https://docs.rs/az/~1.2/az/trait.OverflowingCastFrom.html
+[scf-1-2]: https://docs.rs/az/~1.2/az/trait.SaturatingCastFrom.html
+[ucf-1-2]: https://docs.rs/az/~1.2/az/trait.UnwrappedCastFrom.html
+[wcf-1-2]: https://docs.rs/az/~1.2/az/trait.WrappingCastFrom.html
+
+### Other releases
+
+Details on other releases can be found in [*RELEASES.md*].
+
+[*RELEASES.md*]: https://gitlab.com/tspiteri/az/blob/master/RELEASES.md
+
+## Quick examples
+
+```rust
+use az::{Az, OverflowingAs, WrappingAs};
+use core::num::Wrapping;
+
+// Panics on overflow with `debug_assertions`, otherwise wraps
+assert_eq!(12i32.az::<u32>(), 12u32);
+
+// Always wraps
+let wrapped = 1u32.wrapping_neg();
+assert_eq!((-1).wrapping_as::<u32>(), wrapped);
+assert_eq!((-1).overflowing_as::<u32>(), (wrapped, true));
+
+// Wrapping can also be obtained using `Wrapping`
+assert_eq!((-1).az::<Wrapping<u32>>().0, wrapped);
+```
+
+Conversions from floating-point to integers are also supported.
+Numbers are rounded towards zero, but the [`Round`] wrapper can be
+used to convert floating-point numbers to integers with rounding to
+the nearest, with ties rounded to even.
+
+```rust
+use az::{Az, CheckedAs, Round, SaturatingAs};
+use core::f32;
+
+assert_eq!(15.7.az::<i32>(), 15);
+assert_eq!(Round(15.5).az::<i32>(), 16);
+assert_eq!(1.5e20.saturating_as::<i32>(), i32::max_value());
+assert_eq!(f32::NAN.checked_as::<i32>(), None);
+```
+
+## Implementing casts for other types
+
+To provide casts for another type, you should implement the [`Cast`]
+trait and if necessary the [`CheckedCast`], [`SaturatingCast`],
+[`WrappingCast`], [`OverflowingCast`] and [`UnwrappedCast`] traits.
+The [`Az`], [`CheckedAs`], [`SaturatingAs`], [`WrappingAs`],
+[`OverflowingAs`] and [`UnwrappedAs`] traits are already implemented
+for all types using blanket implementations that make use of the
+former traits.
+
+The cast traits can also be implemented for references. This can be
+useful for expensive types that are not [`Copy`]. For example if you
+have your own integer type that does not implement [`Copy`], you could
+implement casts like in the following example. (`I` could be an
+expensive type, for example a bignum integer, but for the example it
+is only a wrapped [`i32`].)
+
+```rust
+use az::{Az, Cast};
+use core::borrow::Borrow;
+
+struct I(i32);
+impl Cast<i64> for &'_ I {
+    fn cast(self) -> i64 { self.0.cast() }
+}
+
+let owned = I(12);
+assert_eq!(owned.borrow().az::<i64>(), 12);
+```
+
+## Using the *az* crate
+
+The *az* crate is available on [crates.io][*az* crate]. To use it in
+your crate, add it as a dependency inside [*Cargo.toml*]:
+
+```toml
+[dependencies]
+az = "1.2"
+```
+
+The crate requires rustc version 1.31.0 or later.
+
+## License
+
+This crate is free software: you can redistribute it and/or modify it
+under the terms of either
+
+  * the [Apache License, Version 2.0][LICENSE-APACHE] or
+  * the [MIT License][LICENSE-MIT]
+
+at your option.
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally
+submitted for inclusion in the work by you, as defined in the Apache
+License, Version 2.0, shall be dual licensed as above, without any
+additional terms or conditions.
+
+[*Cargo.toml*]: https://doc.rust-lang.org/cargo/guide/dependencies.html
+[*az* crate]: https://crates.io/crates/az
+[LICENSE-APACHE]: https://www.apache.org/licenses/LICENSE-2.0
+[LICENSE-MIT]: https://opensource.org/licenses/MIT
+[`Az`]: https://docs.rs/az/~1.2/az/trait.Az.html
+[`Cast`]: https://docs.rs/az/~1.2/az/trait.Cast.html
+[`CheckedAs`]: https://docs.rs/az/~1.2/az/trait.CheckedAs.html
+[`CheckedCast`]: https://docs.rs/az/~1.2/az/trait.CheckedCast.html
+[`Copy`]: https://doc.rust-lang.org/nightly/core/marker/trait.Copy.html
+[`OverflowingAs`]: https://docs.rs/az/~1.2/az/trait.OverflowingAs.html
+[`OverflowingCast`]: https://docs.rs/az/~1.2/az/trait.OverflowingCast.html
+[`Round`]: https://docs.rs/az/~1.2/az/struct.Round.html
+[`SaturatingAs`]: https://docs.rs/az/~1.2/az/trait.SaturatingAs.html
+[`SaturatingCast`]: https://docs.rs/az/~1.2/az/trait.SaturatingCast.html
+[`UnwrappedAs`]: https://docs.rs/az/~1.2/az/trait.UnwrappedAs.html
+[`UnwrappedCast`]: https://docs.rs/az/~1.2/az/trait.UnwrappedCast.html
+[`WrappingAs`]: https://docs.rs/az/~1.2/az/trait.WrappingAs.html
+[`WrappingCast`]: https://docs.rs/az/~1.2/az/trait.WrappingCast.html
+[`i32`]: https://doc.rust-lang.org/nightly/std/primitive.i32.html