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-0.20.11/tests/enums_unit.rs

@@ -0,0 +1,123 @@
+//! Test expansion of enum variants which have no associated data.
+
+use darling::{ast::NestedMeta, FromMeta};
+use syn::{parse_quote, spanned::Spanned, Meta};
+
+#[derive(Debug, FromMeta)]
+#[darling(rename_all = "snake_case")]
+enum Pattern {
+    Owned,
+    Immutable,
+    Mutable,
+}
+
+#[test]
+fn expansion() {}
+
+#[test]
+fn rejected_in_unit_enum_variants() {
+    #[derive(Debug, FromMeta, PartialEq)]
+    struct Opts {
+        choices: Choices,
+    }
+
+    #[derive(Debug, PartialEq)]
+    struct Choices {
+        values: Vec<Choice>,
+    }
+
+    impl FromMeta for Choices {
+        fn from_list(items: &[NestedMeta]) -> darling::Result<Self> {
+            let values = items
+                .iter()
+                .map(|item| match item {
+                    NestedMeta::Meta(meta) => match meta {
+                        Meta::Path(path) => Choice::from_string(
+                            &path
+                                .get_ident()
+                                .ok_or(
+                                    darling::Error::custom("choice must be an ident (no colon)")
+                                        .with_span(path),
+                                )?
+                                .to_string(),
+                        )
+                        .map_err(|e| e.with_span(path)),
+                        Meta::List(list) => Choice::from_list(&[item.clone()])
+                            .map_err(|e| e.with_span(&list.span())),
+                        Meta::NameValue(n) => Err(darling::Error::custom(
+                            "choice options are not set as name-value, use parentheses",
+                        )
+                        .with_span(&n.eq_token)),
+                    },
+                    _ => {
+                        Err(darling::Error::custom("literal is not a valid choice").with_span(item))
+                    }
+                })
+                .collect::<Result<_, _>>()?;
+            Ok(Self { values })
+        }
+    }
+
+    #[derive(Debug, FromMeta, PartialEq)]
+    enum Choice {
+        One(One),
+        Other,
+    }
+
+    #[derive(Debug, FromMeta, PartialEq)]
+    struct One {
+        foo: String,
+    }
+
+    for (tokens, expected) in [
+        (
+            parse_quote! {
+                choices(one(foo = "bar"))
+            },
+            Ok(Opts {
+                choices: Choices {
+                    values: vec![Choice::One(One {
+                        foo: "bar".to_string(),
+                    })],
+                },
+            }),
+        ),
+        (
+            parse_quote! {
+                choices(other)
+            },
+            Ok(Opts {
+                choices: Choices {
+                    values: vec![Choice::Other],
+                },
+            }),
+        ),
+        (
+            parse_quote! {
+                choices(other, one(foo = "bar"))
+            },
+            Ok(Opts {
+                choices: Choices {
+                    values: vec![
+                        Choice::Other,
+                        Choice::One(One {
+                            foo: "bar".to_string(),
+                        }),
+                    ],
+                },
+            }),
+        ),
+        (
+            parse_quote! {
+                choices(other(foo = "bar"))
+            },
+            Err("Unexpected meta-item format `non-path` at choices".to_string()),
+        ),
+    ] {
+        assert_eq!(
+            Opts::from_list(&NestedMeta::parse_meta_list(tokens).unwrap())
+                .map_err(|e| e.to_string()),
+            expected
+        )
+    }
+}