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/half/README.md

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+# `f16` and `bf16` floating point types for Rust
+[![Crates.io](https://img.shields.io/crates/v/half.svg)](https://crates.io/crates/half/) [![Documentation](https://docs.rs/half/badge.svg)](https://docs.rs/half/) ![Crates.io](https://img.shields.io/crates/l/half) [![Build status](https://github.com/VoidStarKat/half-rs/actions/workflows/rust.yml/badge.svg?branch=main&event=push)](https://github.com/VoidStarKat/half-rs/actions/workflows/rust.yml) [![CircleCI](https://dl.circleci.com/status-badge/img/gh/VoidStarKat/half-rs/tree/main.svg?style=svg)](https://dl.circleci.com/status-badge/redirect/gh/VoidStarKat/half-rs/tree/main)
+
+This crate implements a half-precision floating point `f16` type for Rust implementing the IEEE
+754-2008 standard [`binary16`](https://en.wikipedia.org/wiki/Half-precision_floating-point_format)
+a.k.a "half" format, as well as a `bf16` type implementing the
+[`bfloat16`](https://en.wikipedia.org/wiki/Bfloat16_floating-point_format) format.
+
+## Usage
+
+The `f16` and `bf16` types attempt to match existing Rust floating point type functionality where possible, and provides both conversion operations (such as to/from `f32` and `f64`) and basic
+arithmetic operations. Hardware support for these operations will be used whenever hardware support
+is available—either through instrinsics or targeted assembly—although a nightly Rust toolchain may
+be required for some hardware.
+
+This crate provides [`no_std`](https://rust-embedded.github.io/book/intro/no-std.html) support so can easily be used in embedded code where a smaller float format is most useful.
+
+*Requires Rust 1.81 or greater.* If you need support for older versions of Rust, use previous 
+versions of this crate.
+
+See the [crate documentation](https://docs.rs/half/) for more details.
+
+### Optional Features
+
+- **`alloc`** — Enable use of the [`alloc`](https://doc.rust-lang.org/alloc/) crate when not using
+  the `std` library.
+
+  This enables the `vec` module, which contains zero-copy conversions for the `Vec` type. This
+  allows fast conversion between raw `Vec<u16>` bits and `Vec<f16>` or `Vec<bf16>` arrays, and vice
+  versa.
+
+- **`std`** — Enable features that depend on the Rust `std` library, including everything in the
+  `alloc` feature.
+
+  Enabling the `std` feature enables runtime CPU feature detection of hardware support.
+  Without this feature detection, harware is only used when compiler target supports them.
+
+- **`serde`** - Implement `Serialize` and `Deserialize` traits for `f16` and `bf16`. This adds a
+  dependency on the [`serde`](https://crates.io/crates/serde) crate.
+
+- **`num-traits`** — Enable `ToPrimitive`, `FromPrimitive`, `ToBytes`, `FromBytes`, `Num`, `Float`,
+  `FloatCore`, `Signed`, and `Bounded` trait implementations from the
+  [`num-traits`](https://crates.io/crates/num-traits) crate.
+
+- **`bytemuck`** — Enable `Zeroable` and `Pod` trait implementations from the
+  [`bytemuck`](https://crates.io/crates/bytemuck) crate.
+
+- **`rand_distr`** — Enable sampling from distributions like `StandardUniform` and `StandardNormal` 
+  from the [`rand_distr`](https://crates.io/crates/rand_distr) crate.
+
+- **`rkyv`** -- Enable zero-copy deserializtion with [`rkyv`](https://crates.io/crates/rkyv) crate.
+
+- **`aribtrary`** -- Enable fuzzing support with [`arbitrary`](https://crates.io/crates/arbitrary) 
+  crate by implementing `Arbitrary` trait.
+
+- **`nightly`** -- Enable nightly-only features (currently `loongarch64` intrinsics).
+
+### Hardware support
+
+The following list details hardware support for floating point types in this crate. When using `std`
+library, runtime CPU target detection will be used. To get the most performance benefits, compile
+for specific CPU features which avoids the runtime overhead and works in a `no_std` environment.
+
+| Architecture | CPU Target Feature | Notes                                                                                                                                                  |
+| ------------ | ------------------ |--------------------------------------------------------------------------------------------------------------------------------------------------------|
+| `x86`/`x86_64` | `f16c` | This supports conversion to/from `f16` only (including vector SIMD) and does not support any `bf16` or arithmetic operations.                          |
+| `aarch64` | `fp16` | This supports all operations on `f16` only.                                                                                                            |
+| `loongarch64` | `lsx` | (`nightly` feature only) This supports conversion to/from `f16` only (including vector SIMD) and does not support any `bf16` or arithmetic operations. |
+
+### More Documentation
+
+- [Crate API Reference](https://docs.rs/half/)
+- [Latest Changes](CHANGELOG.md)
+
+## License
+
+All files in this library are dual-licensed and distributed under the terms of either of:
+
+* [MIT License](LICENSE-MIT)
+  ([http://opensource.org/licenses/MIT](http://opensource.org/licenses/MIT))
+* [Apache License, Version 2.0](LICENSE-APACHE)
+  ([http://www.apache.org/licenses/LICENSE-2.0](http://www.apache.org/licenses/LICENSE-2.0))
+
+at your option.
+
+### Contributing
+
+Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the
+work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any
+additional terms or conditions.