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/accrue_errors.rs

@@ -0,0 +1,105 @@
+#![allow(dead_code)]
+//! These tests verify that multiple errors will be collected up from throughout
+//! the parsing process and returned correctly to the caller.
+
+use darling::{ast, FromDeriveInput, FromField, FromMeta};
+use syn::parse_quote;
+
+#[derive(Debug, FromDeriveInput)]
+#[darling(attributes(accrue))]
+struct Lorem {
+    ipsum: String,
+    dolor: Dolor,
+    data: ast::Data<(), LoremField>,
+}
+
+#[derive(Debug, FromMeta)]
+struct Dolor {
+    sit: bool,
+}
+
+#[derive(Debug, FromField)]
+#[darling(attributes(accrue))]
+struct LoremField {
+    ident: Option<syn::Ident>,
+    aliased_as: syn::Ident,
+}
+
+#[test]
+fn bad_type_and_missing_fields() {
+    let input = parse_quote! {
+        #[accrue(ipsum = true, dolor(amet = "Hi"))]
+        pub struct NonConforming {
+            foo: ()
+        }
+    };
+
+    let s_result: ::darling::Error = Lorem::from_derive_input(&input).unwrap_err();
+    let err = s_result.flatten();
+    println!("{}", err);
+    //   3  errors from the struct-level `accrue` attribute
+    // + 1  for the field `foo` missing `aliased_as`
+    // = 4  total errors
+    assert_eq!(4, err.len());
+}
+
+#[test]
+fn body_only_issues() {
+    let input = parse_quote! {
+        #[accrue(ipsum = "Hello", dolor(sit))]
+        pub struct NonConforming {
+            foo: (),
+            bar: bool,
+        }
+    };
+
+    let s_err = Lorem::from_derive_input(&input).unwrap_err();
+    println!("{:?}", s_err);
+    assert_eq!(2, s_err.len());
+}
+
+#[derive(Debug, FromMeta)]
+enum Week {
+    Monday,
+    Tuesday { morning: bool, afternoon: String },
+    Wednesday(Dolor),
+}
+
+#[derive(Debug, FromDeriveInput)]
+#[darling(attributes(accrue))]
+struct Month {
+    schedule: Week,
+}
+
+#[test]
+fn error_in_enum_fields() {
+    let input = parse_quote! {
+        #[accrue(schedule(tuesday(morning = "yes")))]
+        pub struct NonConforming {
+            foo: (),
+            bar: bool,
+        }
+    };
+
+    let s_err = Month::from_derive_input(&input).unwrap_err();
+    assert_eq!(2, s_err.len());
+    let err = s_err.flatten();
+    // TODO add tests to check location path is correct
+    println!("{}", err);
+}
+
+#[test]
+fn error_in_newtype_variant() {
+    let input = parse_quote! {
+        #[accrue(schedule(wednesday(sit = "yes")))]
+        pub struct NonConforming {
+            foo: (),
+            bar: bool,
+        }
+    };
+
+    let s_err = Month::from_derive_input(&input).unwrap_err();
+    assert_eq!(1, s_err.len());
+    println!("{}", s_err);
+    println!("{}", s_err.flatten());
+}