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_expr.rs

@@ -0,0 +1,32 @@
+use darling::FromMeta;
+
+/// This usage of `from_expr` is VALID because there are no unit variants that should conflict with the
+/// implementation.
+#[derive(FromMeta)]
+#[darling(from_expr = |expr| Ok(HasNoUnits::Variant2 { other: format!("{:?}", expr) }))]
+enum HasNoUnits {
+    Variant1 { field: String },
+    Variant2 { other: String },
+}
+
+/// This usage of `from_expr` is invalid because unit variants already generate a from_expr
+/// method, and we don't allow using the from_expr override when it conflicts with the macro's
+/// "normal" operation.
+#[derive(FromMeta)]
+#[darling(from_expr = |expr| Ok(HasUnits::Variant2))]
+enum HasUnits {
+    Variant1 { field: String },
+    Variant2,
+}
+
+fn newtype_from_expr(_expr: &syn::Expr) -> darling::Result<Newtype> {
+    Ok(Newtype(true))
+}
+
+// This usage of `from_expr` is invalid because newtype structs call the inner type's `from_meta`
+// directly from their `from_meta`, so the custom `from_expr` will never be called in normal usage.
+#[derive(FromMeta)]
+#[darling(from_expr = newtype_from_expr)]
+struct Newtype(bool);
+
+fn main() {}