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/no_std.rs

@@ -0,0 +1,28 @@
+extern crate autocfg;
+
+use std::env;
+
+mod support;
+
+/// Tests that we can control the use of `#![no_std]`.
+#[test]
+fn no_std() {
+    // Clear the CI `TARGET`, if any, so we're just dealing with the
+    // host target which always has `std` available.
+    env::remove_var("TARGET");
+
+    // Use the same path as this test binary.
+    let out = support::out_dir();
+
+    let mut ac = autocfg::AutoCfg::with_dir(out.as_ref()).unwrap();
+    assert!(!ac.no_std());
+    assert!(ac.probe_path("std::mem"));
+
+    // `#![no_std]` was stabilized in Rust 1.6
+    if ac.probe_rustc_version(1, 6) {
+        ac.set_no_std(true);
+        assert!(ac.no_std());
+        assert!(!ac.probe_path("std::mem"));
+        assert!(ac.probe_path("core::mem"));
+    }
+}