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/darling/tests/compile-fail/from_word.rs

@@ -0,0 +1,30 @@
+use darling::FromMeta;
+
+// This usage of `from_word` is invalid because unit structs already generate a from_word
+// method, and we don't allow using the from_word override when it conflicts with the macro's
+// "normal" operation.
+#[derive(FromMeta)]
+#[darling(from_word = || Ok(Unit))]
+struct Unit;
+
+fn newtype_from_word() -> darling::Result<Newtype> {
+    Ok(Newtype(true))
+}
+
+// This usage of `from_word` is invalid because newtype structs call the inner type's `from_meta`
+// directly from their `from_meta`, so the custom `from_word` will never be called in normal usage.
+#[derive(FromMeta)]
+#[darling(from_word = newtype_from_word)]
+struct Newtype(bool);
+
+#[derive(FromMeta)]
+#[darling(from_word = || Ok(Wordy::Options { thing: "Hello".to_string() }))]
+enum Wordy {
+    #[darling(word)]
+    Normal,
+    Options {
+        thing: String,
+    },
+}
+
+fn main() {}