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/void/.cargo-checksum.json

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+{"files":{".travis.yml":"ad9b1a707a5c6bcc7c43fddb17a76f633893b0e6fa6891d99415704ae5ca58c2","Cargo.toml":"ea686f87a150a8e43c4b7db57c56d3eda2a4963420d5570d91d99d7d610dd3fb","README.md":"f85783a6fcf9ecc19edabd710775a88430d9e886f46728bfd7d65cef55ff3e73","src/lib.rs":"7ab8269f30715c0729b0e04e5a09be4c413664dc4b530746ea3240ac80a64c66"},"package":"6a02e4885ed3bc0f2de90ea6dd45ebcbb66dacffe03547fadbb0eeae2770887d"}

vendor/void/.travis.yml

@@ -0,0 +1,26 @@
+language: rust
+sudo: false
+
+rust:
+    - 1.0.0
+    - stable
+    - beta
+    - nightly
+
+script:
+    - cargo build
+    - cargo test
+    - if [ "$TRAVIS_RUST_VERSION" != "1.0.0" ]; then cargo test --no-default-features; fi
+    - cargo bench --no-run
+    - cargo doc
+
+after_success:
+    - if [ "$TRAVIS_PULL_REQUEST" == false && test == "TRAVIS_BRANCH" == "master" ]; then
+    - curl https://raw.githubusercontent.com/reem/rust-gh-docs/master/make-docs.sh > docs.sh
+    - chmod u+x docs.sh
+    - ./docs.sh reem project-name
+
+env:
+  global:
+    secure: QPYL1XUr4CyK/2DXlsYC1eCpWRpyEiqQSd/FFVR+YdP/rOJ7AyAXQqPhfgjDBQwvc6E2fUiyYjoV/xe1a757DDeZKlgd8Lp20fSDwvNt/Ejx8ueh3h3kuOtgDpIGSKX/l+XC+ltDpzjhh7bowI2/fOEf+kE53jvu9i4PiLnKdlY=
+

vendor/void/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+
+name = "void"
+version = "1.0.2"
+authors = ["Jonathan Reem <jonathan.reem@gmail.com>"]
+repository = "https://github.com/reem/rust-void.git"
+description = "The uninhabited void type for use in statically impossible cases."
+readme = "README.md"
+license = "MIT"
+
+[features]
+
+default = ["std"]
+std = []
+

vendor/void/README.md

@@ -0,0 +1,39 @@
+# Void
+
+> The uninhabited void type for use in statically impossible cases.
+
+## [Documentation](https://crates.fyi/crates/void/1.0.1)
+
+The uninhabited type, `enum Void { }` is useful in dealing with cases you
+know to be impossible. For instance, if you are implementing a trait which
+allows for error checking, but your case always succeeds, you can mark the
+error case or type as `Void`, signaling to the compiler it can never happen.
+
+This crate also comes packed with a few traits offering extension methods to
+`Result<T, Void>` and `Result<Void, T>`.
+
+## Usage
+
+Use the crates.io repository; add this to your `Cargo.toml` along
+with the rest of your dependencies:
+
+```toml
+[dependencies]
+void = "1"
+```
+
+Then, use `Void` in your crate:
+
+```rust
+extern crate void;
+use void::Void;
+```
+
+## Author
+
+[Jonathan Reem](https://medium.com/@jreem) is the primary author and maintainer of void.
+
+## License
+
+MIT
+

vendor/void/src/lib.rs

@@ -0,0 +1,121 @@
+#![cfg_attr(test, deny(warnings))]
+#![deny(missing_docs)]
+#![cfg_attr(not(feature = "std"), no_std)]
+
+//! # Void
+//!
+//! The uninhabited void type for use in statically impossible cases.
+//!
+//! In its own crate so all the users in the ecosystem can share the same type.
+//! This crate also comes ready with several extension traits for Result that add
+//! extra functionality to `Result<T, Void>` and `Result<Void, E>`.
+//!
+
+#[cfg(not(feature = "std"))]
+mod coreprovider {
+    extern crate core;
+    pub use core::{fmt, cmp};
+}
+
+#[cfg(feature = "std")]
+mod coreprovider {
+    pub use std::{fmt, cmp, error};
+}
+
+use coreprovider::*;
+
+/// The empty type for cases which can't occur.
+#[derive(Copy)]
+pub enum Void { }
+
+impl Clone for Void {
+    fn clone(&self) -> Void {
+        unreachable(*self)
+    }
+}
+
+impl fmt::Debug for Void {
+    fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
+        unreachable(*self)
+    }
+}
+
+impl fmt::Display for Void {
+    fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
+        unreachable(*self)
+    }
+}
+
+impl<T> cmp::PartialEq<T> for Void {
+    fn eq(&self, _: &T) -> bool {
+        unreachable(*self)
+    }
+}
+
+impl<T> cmp::PartialOrd<T> for Void {
+    fn partial_cmp(&self, _: &T) -> Option<cmp::Ordering> {
+        unreachable(*self)
+    }
+}
+
+#[cfg(feature = "std")]
+impl error::Error for Void {
+    fn description(&self) -> &str {
+        unreachable(*self)
+    }
+
+    fn cause(&self) -> Option<&error::Error> {
+        unreachable(*self)
+    }
+}
+
+/// A safe version of `intrinsincs::unreachable`.
+///
+/// If this typechecks, anything that causes this to run is unreachable code.
+///
+/// Calling this function in reachable code invokes undefined behavior, but
+/// should not be possible unless `unsafe` was used elsewhere to construct
+/// an instance of `Void` (which is already undefined behavior).
+#[inline(always)]
+pub fn unreachable(x: Void) -> ! {
+    match x {}
+}
+
+/// Extensions to `Result<T, Void>`
+pub trait ResultVoidExt<T>: Sized {
+    /// Get the value out of a wrapper.
+    fn void_unwrap(self) -> T;
+}
+
+impl<T> ResultVoidExt<T> for Result<T, Void> {
+    /// Get the value out of an always-ok Result.
+    ///
+    /// Never panics, since it is statically known to be Ok.
+    #[inline]
+    fn void_unwrap(self) -> T {
+        match self {
+            Ok(val) => val,
+            Err(e) => unreachable(e)
+        }
+    }
+}
+
+/// Extensions to `Result<Void, E>`
+pub trait ResultVoidErrExt<E>: Sized {
+    /// Get the error out of a wrapper.
+    fn void_unwrap_err(self) -> E;
+}
+
+impl<E> ResultVoidErrExt<E> for Result<Void, E> {
+    /// Get the error out of an always-err Result.
+    ///
+    /// Never panics, since it is statically known to be Err.
+    #[inline]
+    fn void_unwrap_err(self) -> E {
+        match self {
+            Ok(v) => unreachable(v),
+            Err(e) => e
+        }
+    }
+}
+