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/byteorder/README.md

@@ -0,0 +1,77 @@
+byteorder
+=========
+This crate provides convenience methods for encoding and decoding
+numbers in either big-endian or little-endian order.
+
+[![Build status](https://github.com/BurntSushi/byteorder/workflows/ci/badge.svg)](https://github.com/BurntSushi/byteorder/actions)
+[![crates.io](https://img.shields.io/crates/v/byteorder.svg)](https://crates.io/crates/byteorder)
+
+Dual-licensed under MIT or the [UNLICENSE](https://unlicense.org/).
+
+
+### Documentation
+
+https://docs.rs/byteorder
+
+
+### Installation
+
+This crate works with Cargo and is on
+[crates.io](https://crates.io/crates/byteorder). Add it to your `Cargo.toml`
+like so:
+
+```toml
+[dependencies]
+byteorder = "1"
+```
+
+If you want to augment existing `Read` and `Write` traits, then import the
+extension methods like so:
+
+```rust
+use byteorder::{ReadBytesExt, WriteBytesExt, BigEndian, LittleEndian};
+```
+
+For example:
+
+```rust
+use std::io::Cursor;
+use byteorder::{BigEndian, ReadBytesExt};
+
+let mut rdr = Cursor::new(vec![2, 5, 3, 0]);
+// Note that we use type parameters to indicate which kind of byte order
+// we want!
+assert_eq!(517, rdr.read_u16::<BigEndian>().unwrap());
+assert_eq!(768, rdr.read_u16::<BigEndian>().unwrap());
+```
+
+### `no_std` crates
+
+This crate has a feature, `std`, that is enabled by default. To use this crate
+in a `no_std` context, add the following to your `Cargo.toml`:
+
+```toml
+[dependencies]
+byteorder = { version = "1", default-features = false }
+```
+
+
+### Minimum Rust version policy
+
+This crate's minimum supported `rustc` version is `1.60.0`.
+
+The current policy is that the minimum Rust version required to use this crate
+can be increased in minor version updates. For example, if `crate 1.0` requires
+Rust 1.20.0, then `crate 1.0.z` for all values of `z` will also require Rust
+1.20.0 or newer. However, `crate 1.y` for `y > 0` may require a newer minimum
+version of Rust.
+
+In general, this crate will be conservative with respect to the minimum
+supported version of Rust.
+
+
+### Alternatives
+
+Note that as of Rust 1.32, the standard numeric types provide built-in methods
+like `to_le_bytes` and `from_le_bytes`, which support some of the same use
+cases.