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/heapless/build.rs

@@ -0,0 +1,92 @@
+#![deny(warnings)]
+
+use std::{
+    env,
+    error::Error,
+    fs,
+    path::Path,
+    process::{Command, ExitStatus, Stdio},
+};
+
+fn main() -> Result<(), Box<dyn Error>> {
+    let target = env::var("TARGET")?;
+
+    // Manually list targets that have atomic load/store, but no CAS.
+    // Remove when `cfg(target_has_atomic_load_store)` is stable.
+    // last updated nightly-2023-10-28
+    match &target[..] {
+        "armv4t-none-eabi"
+        | "armv5te-none-eabi"
+        | "avr-unknown-gnu-atmega328"
+        | "bpfeb-unknown-none"
+        | "bpfel-unknown-none"
+        | "thumbv4t-none-eabi"
+        | "thumbv5te-none-eabi"
+        | "thumbv6m-none-eabi" => println!("cargo:rustc-cfg=has_atomic_load_store"),
+        _ => {}
+    };
+
+    // AArch64 instruction set contains `clrex` but not `ldrex` or `strex`; the
+    // probe will succeed when we already know to deny this target from LLSC.
+    if !target.starts_with("aarch64") {
+        match compile_probe(ARM_LLSC_PROBE) {
+            Some(status) if status.success() => println!("cargo:rustc-cfg=arm_llsc"),
+            _ => {}
+        }
+    }
+
+    Ok(())
+}
+
+const ARM_LLSC_PROBE: &str = r#"
+#![no_std]
+
+// `no_mangle` forces codegen, which makes llvm check the contents of the `asm!` macro
+#[no_mangle]
+unsafe fn asm() {
+    core::arch::asm!("clrex");
+}
+"#;
+
+// this function was taken from anyhow v1.0.63 build script
+// https://crates.io/crates/anyhow/1.0.63 (last visited 2022-09-02)
+// the code is licensed under 'MIT or APACHE-2.0'
+fn compile_probe(source: &str) -> Option<ExitStatus> {
+    let rustc = env::var_os("RUSTC")?;
+    let out_dir = env::var_os("OUT_DIR")?;
+    let probefile = Path::new(&out_dir).join("probe.rs");
+    fs::write(&probefile, source).ok()?;
+
+    // Make sure to pick up Cargo rustc configuration.
+    let mut cmd = if let Some(wrapper) = env::var_os("RUSTC_WRAPPER") {
+        let mut cmd = Command::new(wrapper);
+        // The wrapper's first argument is supposed to be the path to rustc.
+        cmd.arg(rustc);
+        cmd
+    } else {
+        Command::new(rustc)
+    };
+
+    cmd.stderr(Stdio::null())
+        .arg("--edition=2018")
+        .arg("--crate-name=probe")
+        .arg("--crate-type=lib")
+        .arg("--out-dir")
+        .arg(out_dir)
+        .arg(probefile);
+
+    if let Some(target) = env::var_os("TARGET") {
+        cmd.arg("--target").arg(target);
+    }
+
+    // If Cargo wants to set RUSTFLAGS, use that.
+    if let Ok(rustflags) = env::var("CARGO_ENCODED_RUSTFLAGS") {
+        if !rustflags.is_empty() {
+            for arg in rustflags.split('\x1f') {
+                cmd.arg(arg);
+            }
+        }
+    }
+
+    cmd.status().ok()
+}