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/proc-macro-crate/tests/workspace_deps.rs

@@ -0,0 +1,17 @@
+use std::{path::PathBuf, process::Command};
+
+#[test]
+fn workspace_deps_working() {
+    let manifest_dir =
+        PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("tests/workspace_deps/Cargo.toml");
+
+    assert!(Command::new("cargo")
+        .arg("build")
+        .arg("--all")
+        .arg(format!("--manifest-path={}", manifest_dir.display()))
+        .spawn()
+        .unwrap()
+        .wait()
+        .unwrap()
+        .success());
+}