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/ssd1306/README.md

@@ -0,0 +1,108 @@
+# SSD1306 driver
+
+[![Build Status](https://github.com/rust-embedded-community/ssd1306/actions/workflows/ci.yml/badge.svg)](https://github.com/rust-embedded-community/ssd1306/actions/workflows/ci.yml)
+[![Crates.io](https://img.shields.io/crates/v/ssd1306.svg)](https://crates.io/crates/ssd1306)
+[![Docs.rs](https://docs.rs/ssd1306/badge.svg)](https://docs.rs/ssd1306)
+
+[![CRIUS display showing the Rust logo](readme_banner.jpg?raw=true)](examples/image_i2c.rs)
+
+I2C and SPI (4 wire) driver for the SSD1306 OLED display.
+
+## [Documentation](https://docs.rs/ssd1306)
+
+## [Changelog](CHANGELOG.md)
+
+## [Examples](examples)
+
+This crate uses [`probe-run`](https://crates.io/crates/probe-run) to run the examples. Once set up,
+it should be as simple as `cargo run --example <example name> --release`.
+
+From [`examples/image_i2c.rs`](examples/image_i2c.rs):
+
+```rust
+#![no_std]
+#![no_main]
+
+use cortex_m_rt::{entry, exception, ExceptionFrame};
+use embedded_graphics::{
+    image::{Image, ImageRaw},
+    pixelcolor::BinaryColor,
+    prelude::*,
+};
+use panic_halt as _;
+use ssd1306::{prelude::*, I2CDisplayInterface, Ssd1306};
+use stm32f1xx_hal::{
+    i2c::{BlockingI2c, DutyCycle, Mode},
+    prelude::*,
+    stm32,
+};
+
+#[entry]
+fn main() -> ! {
+    let dp = stm32::Peripherals::take().unwrap();
+
+    let mut flash = dp.FLASH.constrain();
+    let mut rcc = dp.RCC.constrain();
+
+    let clocks = rcc.cfgr.freeze(&mut flash.acr);
+
+    let mut afio = dp.AFIO.constrain(&mut rcc.apb2);
+
+    let mut gpiob = dp.GPIOB.split(&mut rcc.apb2);
+
+    let scl = gpiob.pb8.into_alternate_open_drain(&mut gpiob.crh);
+    let sda = gpiob.pb9.into_alternate_open_drain(&mut gpiob.crh);
+
+    let i2c = BlockingI2c::i2c1(
+        dp.I2C1,
+        (scl, sda),
+        &mut afio.mapr,
+        Mode::Fast {
+            frequency: 400_000.hz(),
+            duty_cycle: DutyCycle::Ratio2to1,
+        },
+        clocks,
+        &mut rcc.apb1,
+        1000,
+        10,
+        1000,
+        1000,
+    );
+
+    let interface = I2CDisplayInterface::new(i2c);
+    let mut display = Ssd1306::new(interface, DisplaySize128x64, DisplayRotation::Rotate0)
+        .into_buffered_graphics_mode();
+    display.init().unwrap();
+
+    let raw: ImageRaw<BinaryColor> = ImageRaw::new(include_bytes!("./rust.raw"), 64);
+
+    let im = Image::new(&raw, Point::new(32, 0));
+
+    im.draw(&mut display).unwrap();
+
+    display.flush().unwrap();
+
+    loop {}
+}
+
+#[exception]
+fn HardFault(ef: &ExceptionFrame) -> ! {
+    panic!("{:#?}", ef);
+}
+```
+
+## License
+
+Licensed under either of
+
+- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
+  http://www.apache.org/licenses/LICENSE-2.0)
+- MIT license ([LICENSE-MIT](LICENSE-MIT) or http://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.