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/maybe-async-cfg/tests/test.rs

@@ -0,0 +1,15 @@
+#[test]
+fn ui() {
+    let t = trybuild::TestCases::new();
+    t.pass("tests/ui/01-maybe-async.rs");
+    t.pass("tests/ui/02-must-be-async.rs");
+    t.pass("tests/ui/03-must-be-sync.rs");
+    t.pass("tests/ui/04-unit-test-util.rs");
+    t.pass("tests/ui/05-replace-future-generic-type-with-output.rs");
+    t.pass("tests/ui/06-sync_impl_async_impl.rs");
+
+    t.compile_fail("tests/ui/test_fail/01-empty-test.rs");
+    t.compile_fail("tests/ui/test_fail/02-unknown-path.rs");
+    t.compile_fail("tests/ui/test_fail/03-async-gt2.rs");
+    t.compile_fail("tests/ui/test_fail/04-bad-sync-cond.rs");
+}