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/rustc_version/README.md

@@ -0,0 +1,75 @@
+rustc-version-rs
+==============
+
+A library for querying the version of a `rustc` compiler.
+
+This can be used by build scripts or other tools dealing with Rust sources
+to make decisions based on the version of the compiler.
+
+[![Travis-CI Status](https://travis-ci.org/Kimundi/rustc-version-rs.png?branch=master)](https://travis-ci.org/Kimundi/rustc-version-rs)
+
+# Getting Started
+
+[rustc-version-rs is available on crates.io](https://crates.io/crates/rustc_version).
+It is recommended to look there for the newest released version, as well as links to the newest builds of the docs.
+
+At the point of the last update of this README, the latest published version could be used like this:
+
+Add the following dependency to your Cargo manifest...
+
+```toml
+[build-dependencies]
+rustc_version = "0.2"
+```
+
+...and see the [docs](http://kimundi.github.io/rustc-version-rs/rustc_version/index.html) for how to use it.
+
+# Example
+
+```rust
+// This could be a cargo build script
+
+extern crate rustc_version;
+use rustc_version::{version, version_meta, Channel, Version};
+
+fn main() {
+    // Assert we haven't travelled back in time
+    assert!(version().unwrap().major >= 1);
+
+    // Set cfg flags depending on release channel
+    match version_meta().unwrap().channel {
+        Channel::Stable => {
+            println!("cargo:rustc-cfg=RUSTC_IS_STABLE");
+        }
+        Channel::Beta => {
+            println!("cargo:rustc-cfg=RUSTC_IS_BETA");
+        }
+        Channel::Nightly => {
+            println!("cargo:rustc-cfg=RUSTC_IS_NIGHTLY");
+        }
+        Channel::Dev => {
+            println!("cargo:rustc-cfg=RUSTC_IS_DEV");
+        }
+    }
+
+    // Check for a minimum version
+    if version().unwrap() >= Version::parse("1.4.0").unwrap() {
+        println!("cargo:rustc-cfg=compiler_has_important_bugfix");
+    }
+}
+```
+
+## 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.