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/hashbrown/README.md

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+hashbrown
+=========
+
+[![Build Status](https://github.com/rust-lang/hashbrown/actions/workflows/rust.yml/badge.svg)](https://github.com/rust-lang/hashbrown/actions)
+[![Crates.io](https://img.shields.io/crates/v/hashbrown.svg)](https://crates.io/crates/hashbrown)
+[![Documentation](https://docs.rs/hashbrown/badge.svg)](https://docs.rs/hashbrown)
+[![Rust](https://img.shields.io/badge/rust-1.65.0%2B-blue.svg?maxAge=3600)](https://github.com/rust-lang/hashbrown)
+
+This crate is a Rust port of Google's high-performance [SwissTable] hash
+map, adapted to make it a drop-in replacement for Rust's standard `HashMap`
+and `HashSet` types.
+
+The original C++ version of SwissTable can be found [here], and this
+[CppCon talk] gives an overview of how the algorithm works.
+
+Since Rust 1.36, this is now the `HashMap` implementation for the Rust standard
+library. However you may still want to use this crate instead since it works
+in environments without `std`, such as embedded systems and kernels.
+
+[SwissTable]: https://abseil.io/blog/20180927-swisstables
+[here]: https://github.com/abseil/abseil-cpp/blob/master/absl/container/internal/raw_hash_set.h
+[CppCon talk]: https://www.youtube.com/watch?v=ncHmEUmJZf4
+
+## [Change log](CHANGELOG.md)
+
+## Features
+
+- Drop-in replacement for the standard library `HashMap` and `HashSet` types.
+- Uses [foldhash](https://github.com/orlp/foldhash) as the default hasher, which is much faster than SipHash.
+  However, foldhash does *not provide the same level of HashDoS resistance* as SipHash, so if that is important to you, you might want to consider using a different hasher.
+- Around 2x faster than the previous standard library `HashMap`.
+- Lower memory usage: only 1 byte of overhead per entry instead of 8.
+- Compatible with `#[no_std]` (but requires a global allocator with the `alloc` crate).
+- Empty hash maps do not allocate any memory.
+- SIMD lookups to scan multiple hash entries in parallel.
+
+## Usage
+
+Add this to your `Cargo.toml`:
+
+```toml
+[dependencies]
+hashbrown = "0.16"
+```
+
+Then:
+
+```rust
+use hashbrown::HashMap;
+
+let mut map = HashMap::new();
+map.insert(1, "one");
+```
+
+## Flags
+This crate has the following Cargo features:
+
+- `nightly`: Enables nightly-only features including: `#[may_dangle]`.
+- `serde`: Enables serde serialization support.
+- `rayon`: Enables rayon parallel iterator support.
+- `equivalent`: Allows comparisons to be customized with the `Equivalent` trait. (enabled by default)
+- `raw-entry`: Enables access to the deprecated `RawEntry` API.
+- `inline-more`: Adds inline hints to most functions, improving run-time performance at the cost
+  of compilation time. (enabled by default)
+- `default-hasher`: Compiles with foldhash as default hasher. (enabled by default)
+- `allocator-api2`: Enables support for allocators that support `allocator-api2`. (enabled by default)
+
+## License
+
+Licensed under either of:
+
+ * Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or https://www.apache.org/licenses/LICENSE-2.0)
+ * MIT license ([LICENSE-MIT](LICENSE-MIT) or https://opensource.org/licenses/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-2.0 license, shall be dual licensed as above, without any
+additional terms or conditions.