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/document-features/tests/self-doc.rs

@@ -0,0 +1,37 @@
+#[test]
+fn ensure_it_compiles() {
+    document_features::document_features!();
+    document_features::document_features!(feature_label = "**`{feature}`**");
+    document_features::document_features!(feature_label = r"**`{feature}`**");
+    document_features::document_features!(feature_label = r#"**`{feature}`**"#);
+    document_features::document_features!(
+        feature_label = "<span class=\"stab portability\"><code>{feature}</code></span>"
+    );
+    document_features::document_features!(
+        feature_label = r#"<span class="stab portability"><code>{feature}</code></span>"#
+    );
+    document_features::document_features!(
+        feature_label = r##"<span class="stab portability"><code>{feature}</code></span>"##
+    );
+}
+
+#[test]
+fn self_doc() {
+    let actual = document_features::document_features!();
+    let expected = "* **`self-test`** —  Internal feature used only for the tests, don't enable\n";
+    assert_eq!(actual, expected);
+}
+
+#[test]
+fn self_doc_with_custom_label() {
+    let actual = document_features::document_features!(
+        feature_label = r#"<span class="stab portability"><code>{feature}</code></span>"#
+    );
+    let expected =
+        "* <span class=\"stab portability\"><code>self-test</code></span> —  Internal feature used only for the tests, don't enable\n";
+    assert_eq!(actual, expected);
+    let actual2 = document_features::document_features!(
+        feature_label = "<span class=\"stab\u{0020}portability\"><code>{feature}</code></span>"
+    );
+    assert_eq!(actual2, expected);
+}