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/embassy-executor-macros/src/macros/main.rs

@@ -0,0 +1,177 @@
+use std::str::FromStr;
+
+use darling::export::NestedMeta;
+use darling::FromMeta;
+use proc_macro2::TokenStream;
+use quote::quote;
+use syn::{ReturnType, Type};
+
+use crate::util::*;
+
+enum Flavor {
+    Standard,
+    Wasm,
+}
+
+pub(crate) struct Arch {
+    default_entry: Option<&'static str>,
+    flavor: Flavor,
+}
+
+pub static ARCH_AVR: Arch = Arch {
+    default_entry: Some("avr_device::entry"),
+    flavor: Flavor::Standard,
+};
+
+pub static ARCH_RISCV: Arch = Arch {
+    default_entry: Some("riscv_rt::entry"),
+    flavor: Flavor::Standard,
+};
+
+pub static ARCH_CORTEX_M: Arch = Arch {
+    default_entry: Some("cortex_m_rt::entry"),
+    flavor: Flavor::Standard,
+};
+
+pub static ARCH_SPIN: Arch = Arch {
+    default_entry: None,
+    flavor: Flavor::Standard,
+};
+
+pub static ARCH_STD: Arch = Arch {
+    default_entry: None,
+    flavor: Flavor::Standard,
+};
+
+pub static ARCH_WASM: Arch = Arch {
+    default_entry: Some("wasm_bindgen::prelude::wasm_bindgen(start)"),
+    flavor: Flavor::Wasm,
+};
+
+#[derive(Debug, FromMeta, Default)]
+struct Args {
+    #[darling(default)]
+    entry: Option<String>,
+}
+
+pub fn run(args: TokenStream, item: TokenStream, arch: &Arch) -> TokenStream {
+    let mut errors = TokenStream::new();
+
+    // If any of the steps for this macro fail, we still want to expand to an item that is as close
+    // to the expected output as possible. This helps out IDEs such that completions and other
+    // related features keep working.
+    let f: ItemFn = match syn::parse2(item.clone()) {
+        Ok(x) => x,
+        Err(e) => return token_stream_with_error(item, e),
+    };
+
+    let args = match NestedMeta::parse_meta_list(args) {
+        Ok(x) => x,
+        Err(e) => return token_stream_with_error(item, e),
+    };
+
+    let args = match Args::from_list(&args) {
+        Ok(x) => x,
+        Err(e) => {
+            errors.extend(e.write_errors());
+            Args::default()
+        }
+    };
+
+    let fargs = f.sig.inputs.clone();
+
+    if f.sig.asyncness.is_none() {
+        error(&mut errors, &f.sig, "main function must be async");
+    }
+    if !f.sig.generics.params.is_empty() {
+        error(&mut errors, &f.sig, "main function must not be generic");
+    }
+    if !f.sig.generics.where_clause.is_none() {
+        error(&mut errors, &f.sig, "main function must not have `where` clauses");
+    }
+    if !f.sig.abi.is_none() {
+        error(&mut errors, &f.sig, "main function must not have an ABI qualifier");
+    }
+    if !f.sig.variadic.is_none() {
+        error(&mut errors, &f.sig, "main function must not be variadic");
+    }
+    match &f.sig.output {
+        ReturnType::Default => {}
+        ReturnType::Type(_, ty) => match &**ty {
+            Type::Tuple(tuple) if tuple.elems.is_empty() => {}
+            Type::Never(_) => {}
+            _ => error(
+                &mut errors,
+                &f.sig,
+                "main function must either not return a value, return `()` or return `!`",
+            ),
+        },
+    }
+
+    if fargs.len() != 1 {
+        error(&mut errors, &f.sig, "main function must have 1 argument: the spawner.");
+    }
+
+    let entry = match args.entry.as_deref().or(arch.default_entry) {
+        None => TokenStream::new(),
+        Some(x) => match TokenStream::from_str(x) {
+            Ok(x) => quote!(#[#x]),
+            Err(e) => {
+                error(&mut errors, &f.sig, e);
+                TokenStream::new()
+            }
+        },
+    };
+
+    let f_body = f.body;
+    let out = &f.sig.output;
+
+    let (main_ret, mut main_body) = match arch.flavor {
+        Flavor::Standard => (
+            quote!(!),
+            quote! {
+                unsafe fn __make_static<T>(t: &mut T) -> &'static mut T {
+                    ::core::mem::transmute(t)
+                }
+
+                let mut executor = ::embassy_executor::Executor::new();
+                let executor = unsafe { __make_static(&mut executor) };
+                executor.run(|spawner| {
+                    spawner.must_spawn(__embassy_main(spawner));
+                })
+            },
+        ),
+        Flavor::Wasm => (
+            quote!(Result<(), wasm_bindgen::JsValue>),
+            quote! {
+                let executor = ::std::boxed::Box::leak(::std::boxed::Box::new(::embassy_executor::Executor::new()));
+
+                executor.start(|spawner| {
+                    spawner.must_spawn(__embassy_main(spawner));
+                });
+
+                Ok(())
+            },
+        ),
+    };
+
+    if !errors.is_empty() {
+        main_body = quote! {loop{}};
+    }
+
+    let result = quote! {
+        #[::embassy_executor::task()]
+        async fn __embassy_main(#fargs) #out {
+            #f_body
+        }
+
+        #entry
+        fn main() -> #main_ret {
+            #main_body
+        }
+
+        #errors
+    };
+
+    result
+}