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/defmt/tests/ui.rs

@@ -0,0 +1,15 @@
+#[test]
+fn ui() {
+    // only test error messages on the stable channel (nightly may change too often)
+    if rustc_version::version_meta()
+        .map(|meta| meta.channel == rustc_version::Channel::Stable)
+        .unwrap_or(false)
+        && std::env::var_os("SKIP_UI_TESTS").is_none()
+    {
+        let t = trybuild::TestCases::new();
+        t.compile_fail("tests/ui/*.rs");
+
+        t.pass("tests/basic_usage.rs");
+        t.pass("tests/derive-bounds.rs");
+    }
+}