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/autocfg/tests/rustflags.rs

@@ -0,0 +1,34 @@
+extern crate autocfg;
+
+use std::env;
+
+mod support;
+
+/// Tests that autocfg uses the RUSTFLAGS or CARGO_ENCODED_RUSTFLAGS
+/// environment variables when running rustc.
+#[test]
+fn test_with_sysroot() {
+    let dir = support::exe_dir();
+    let out = support::out_dir();
+
+    // If we have encoded rustflags, they take precedence, even if empty.
+    env::set_var("CARGO_ENCODED_RUSTFLAGS", "");
+    env::set_var("RUSTFLAGS", &format!("-L {}", dir.display()));
+    let ac = autocfg::AutoCfg::with_dir(out.as_ref()).unwrap();
+    assert!(ac.probe_sysroot_crate("std"));
+    assert!(!ac.probe_sysroot_crate("autocfg"));
+
+    // Now try again with useful encoded args.
+    env::set_var(
+        "CARGO_ENCODED_RUSTFLAGS",
+        &format!("-L\x1f{}", dir.display()),
+    );
+    let ac = autocfg::AutoCfg::with_dir(out.as_ref()).unwrap();
+    assert!(ac.probe_sysroot_crate("autocfg"));
+
+    // Try the old-style RUSTFLAGS, ensuring HOST != TARGET.
+    env::remove_var("CARGO_ENCODED_RUSTFLAGS");
+    env::set_var("HOST", "lol");
+    let ac = autocfg::AutoCfg::with_dir(out.as_ref()).unwrap();
+    assert!(ac.probe_sysroot_crate("autocfg"));
+}