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/syn-1.0.109/build.rs

@@ -0,0 +1,51 @@
+use std::env;
+use std::process::Command;
+use std::str;
+
+// The rustc-cfg strings below are *not* public API. Please let us know by
+// opening a GitHub issue if your build environment requires some way to enable
+// these cfgs other than by executing our build script.
+fn main() {
+    let compiler = match rustc_version() {
+        Some(compiler) => compiler,
+        None => return,
+    };
+
+    if compiler.minor < 36 {
+        println!("cargo:rustc-cfg=syn_omit_await_from_token_macro");
+    }
+
+    if compiler.minor < 39 {
+        println!("cargo:rustc-cfg=syn_no_const_vec_new");
+    }
+
+    if compiler.minor < 40 {
+        println!("cargo:rustc-cfg=syn_no_non_exhaustive");
+    }
+
+    if compiler.minor < 56 {
+        println!("cargo:rustc-cfg=syn_no_negative_literal_parse");
+    }
+
+    if !compiler.nightly {
+        println!("cargo:rustc-cfg=syn_disable_nightly_tests");
+    }
+}
+
+struct Compiler {
+    minor: u32,
+    nightly: bool,
+}
+
+fn rustc_version() -> Option<Compiler> {
+    let rustc = env::var_os("RUSTC")?;
+    let output = Command::new(rustc).arg("--version").output().ok()?;
+    let version = str::from_utf8(&output.stdout).ok()?;
+    let mut pieces = version.split('.');
+    if pieces.next() != Some("rustc 1") {
+        return None;
+    }
+    let minor = pieces.next()?.parse().ok()?;
+    let nightly = version.contains("nightly") || version.ends_with("-dev");
+    Some(Compiler { minor, nightly })
+}