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/esp-build/src/lib.rs

@@ -0,0 +1,222 @@
+//! Build utilities for esp-hal.
+
+#![doc(html_logo_url = "https://avatars.githubusercontent.com/u/46717278")]
+
+use std::{io::Write as _, process};
+
+use proc_macro::TokenStream;
+use syn::{parse_macro_input, punctuated::Punctuated, LitStr, Token};
+use termcolor::{Color, ColorChoice, ColorSpec, StandardStream, WriteColor};
+
+/// Print a build error and terminate the process.
+///
+/// It should be noted that the error will be printed BEFORE the main function
+/// is called, and as such this should NOT be thought analogous to `println!` or
+/// similar utilities.
+///
+/// ## Example
+///
+/// ```rust
+/// esp_build::error! {"
+/// ERROR: something really bad has happened!
+/// "}
+/// // Process exits with exit code 1
+/// ```
+#[proc_macro]
+pub fn error(input: TokenStream) -> TokenStream {
+    do_alert(Color::Red, input);
+    process::exit(1);
+}
+
+/// Print a build warning.
+///
+/// It should be noted that the warning will be printed BEFORE the main function
+/// is called, and as such this should NOT be thought analogous to `println!` or
+/// similar utilities.
+///
+/// ## Example
+///
+/// ```rust
+/// esp_build::warning! {"
+/// WARNING: something unpleasant has happened!
+/// "};
+/// ```
+#[proc_macro]
+pub fn warning(input: TokenStream) -> TokenStream {
+    do_alert(Color::Yellow, input)
+}
+
+/// Given some features, assert that **at most** one of the features is enabled.
+///
+/// ## Example
+/// ```rust
+/// assert_unique_features!("foo", "bar", "baz");
+/// ```
+#[proc_macro]
+pub fn assert_unique_features(input: TokenStream) -> TokenStream {
+    let features = parse_macro_input!(input with Punctuated<LitStr, Token![,]>::parse_terminated)
+        .into_iter()
+        .collect::<Vec<_>>();
+
+    let pairs = unique_pairs(&features);
+    let unique_cfgs = pairs
+        .iter()
+        .map(|(a, b)| quote::quote! { all(feature = #a, feature = #b) });
+
+    let message = format!(
+        r#"
+ERROR: expected exactly zero or one enabled feature from feature group:
+  {:?}
+"#,
+        features.iter().map(|lit| lit.value()).collect::<Vec<_>>(),
+    );
+
+    quote::quote! {
+        #[cfg(any(#(#unique_cfgs),*))]
+        ::esp_build::error! { #message }
+    }
+    .into()
+}
+
+/// Given some features, assert that **at least** one of the features is
+/// enabled.
+///
+/// ## Example
+/// ```rust
+/// assert_used_features!("foo", "bar", "baz");
+/// ```
+#[proc_macro]
+pub fn assert_used_features(input: TokenStream) -> TokenStream {
+    let features = parse_macro_input!(input with Punctuated<LitStr, Token![,]>::parse_terminated)
+        .into_iter()
+        .collect::<Vec<_>>();
+
+    let message = format!(
+        r#"
+ERROR: expected at least one enabled feature from feature group:
+  {:?}
+    "#,
+        features.iter().map(|lit| lit.value()).collect::<Vec<_>>()
+    );
+
+    quote::quote! {
+        #[cfg(not(any(#(feature = #features),*)))]
+        ::esp_build::error! { #message }
+    }
+    .into()
+}
+
+/// Given some features, assert that **exactly** one of the features is enabled.
+///
+/// ## Example
+/// ```rust
+/// assert_unique_used_features!("foo", "bar", "baz");
+/// ```
+#[proc_macro]
+pub fn assert_unique_used_features(input: TokenStream) -> TokenStream {
+    let features = parse_macro_input!(input with Punctuated<LitStr, Token![,]>::parse_terminated)
+        .into_iter()
+        .collect::<Vec<_>>();
+
+    let pairs = unique_pairs(&features);
+    let unique_cfgs = pairs
+        .iter()
+        .map(|(a, b)| quote::quote! { all(feature = #a, feature = #b) });
+
+    let message = format!(
+        r#"
+ERROR: expected exactly one enabled feature from feature group:
+  {:?}
+    "#,
+        features.iter().map(|lit| lit.value()).collect::<Vec<_>>()
+    );
+
+    quote::quote! {
+        #[cfg(any(any(#(#unique_cfgs),*), not(any(#(feature = #features),*))))]
+        ::esp_build::error! { #message }
+    }
+    .into()
+}
+
+// ----------------------------------------------------------------------------
+// Helper Functions
+
+// Adapted from:
+// https://github.com/dtolnay/build-alert/blob/49d060e/src/lib.rs#L54-L93
+fn do_alert(color: Color, input: TokenStream) -> TokenStream {
+    let message = parse_macro_input!(input as LitStr).value();
+
+    let ref mut stderr = StandardStream::stderr(ColorChoice::Auto);
+    let color_spec = ColorSpec::new().set_fg(Some(color)).clone();
+
+    let mut has_nonspace = false;
+
+    for mut line in message.lines() {
+        if !has_nonspace {
+            let (maybe_heading, rest) = split_heading(line);
+
+            if let Some(heading) = maybe_heading {
+                stderr.set_color(color_spec.clone().set_bold(true)).ok();
+                write!(stderr, "\n{}", heading).ok();
+                has_nonspace = true;
+            }
+
+            line = rest;
+        }
+
+        if line.is_empty() {
+            writeln!(stderr).ok();
+        } else {
+            stderr.set_color(&color_spec).ok();
+            writeln!(stderr, "{}", line).ok();
+
+            has_nonspace = has_nonspace || line.contains(|ch: char| ch != ' ');
+        }
+    }
+
+    stderr.reset().ok();
+    writeln!(stderr).ok();
+
+    TokenStream::new()
+}
+
+// Adapted from:
+// https://github.com/dtolnay/build-alert/blob/49d060e/src/lib.rs#L95-L114
+fn split_heading(s: &str) -> (Option<&str>, &str) {
+    let mut end = 0;
+    while end < s.len() && s[end..].starts_with(|ch: char| ch.is_ascii_uppercase()) {
+        end += 1;
+    }
+
+    if end >= 3 && (end == s.len() || s[end..].starts_with(':')) {
+        let (heading, rest) = s.split_at(end);
+        (Some(heading), rest)
+    } else {
+        (None, s)
+    }
+}
+
+fn unique_pairs(features: &Vec<LitStr>) -> Vec<(&LitStr, &LitStr)> {
+    let mut pairs = Vec::new();
+
+    let mut i = 0;
+    let mut j = 0;
+
+    while i < features.len() {
+        let a = &features[i];
+        let b = &features[j];
+
+        if a.value() != b.value() {
+            pairs.push((a, b));
+        }
+
+        j += 1;
+
+        if j >= features.len() {
+            i += 1;
+            j = i;
+        }
+    }
+
+    pairs
+}