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/unsupported_attributes.rs

@@ -0,0 +1,31 @@
+use darling::FromDeriveInput;
+use syn::{parse_quote, Ident, LitStr, Path};
+
+#[derive(Debug, FromDeriveInput)]
+#[darling(supports(struct_unit), attributes(bar))]
+pub struct Bar {
+    pub ident: Ident,
+    pub st: Path,
+    pub file: LitStr,
+}
+
+/// Per [#96](https://github.com/TedDriggs/darling/issues/96), make sure that an
+/// attribute which isn't a valid meta gets an error.
+/// Properties can be split across multiple attributes; this test ensures that one
+/// non-meta attribute does not interfere with the parsing of other, well-formed attributes.
+#[test]
+fn non_meta_attribute_does_not_block_others() {
+    let di = parse_quote! {
+        #[derive(Bar)]
+        #[bar(st = RocketEngine: Debug)]
+        #[bar(file = "motors/example_6.csv")]
+        pub struct EstesC6;
+    };
+
+    let errors: darling::Error = Bar::from_derive_input(&di).unwrap_err().flatten();
+    // The number of errors here is 2:
+    // - The parsing error caused by a where-clause body where it doesn't belong
+    // - The missing `st` value because the parsing failure blocked that attribute from
+    //   being read.
+    assert_eq!(2, errors.len());
+}