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/flatten_error_accumulation.rs

@@ -0,0 +1,45 @@
+use darling::{util::Flag, Error, FromDeriveInput, FromMeta};
+use proc_macro2::Ident;
+use syn::parse_quote;
+
+#[derive(FromMeta)]
+#[darling(and_then = Self::validate)]
+struct Vis {
+    public: Flag,
+    private: Flag,
+}
+
+impl Vis {
+    fn validate(self) -> darling::Result<Self> {
+        if self.public.is_present() && self.private.is_present() {
+            return Err(Error::custom("Cannot be both public and private"));
+        }
+
+        Ok(self)
+    }
+}
+
+#[derive(FromDeriveInput)]
+#[darling(attributes(sample))]
+#[allow(dead_code)]
+struct Example {
+    ident: Ident,
+    label: String,
+    volume: usize,
+    #[darling(flatten)]
+    visibility: Vis,
+}
+
+#[test]
+fn many_errors() {
+    let e = Example::from_derive_input(&parse_quote! {
+        #[sample(volume = 10, public, private)]
+        struct Demo {}
+    })
+    .map(|_| "Should have failed")
+    .unwrap_err();
+
+    // We are expecting an error from the Vis::validate method and an error for the
+    // missing `label` field.
+    assert_eq!(e.len(), 2);
+}