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/proc-macro-error-attr/src/settings.rs

@@ -0,0 +1,72 @@
+use crate::{Error, Result};
+use proc_macro2::{Ident, Span, TokenStream, TokenTree};
+
+macro_rules! decl_settings {
+    ($($val:expr => $variant:ident),+ $(,)*) => {
+        #[derive(PartialEq)]
+        pub(crate) enum Setting {
+            $($variant),*
+        }
+
+        fn ident_to_setting(ident: Ident) -> Result<Setting> {
+            match &*ident.to_string() {
+                $($val => Ok(Setting::$variant),)*
+                _ => {
+                    let possible_vals = [$($val),*]
+                        .iter()
+                        .map(|v| format!("`{}`", v))
+                        .collect::<Vec<_>>()
+                        .join(", ");
+
+                    Err(Error::new(
+                        ident.span(),
+                        format!("unknown setting `{}`, expected one of {}", ident, possible_vals)))
+                }
+            }
+        }
+    };
+}
+
+decl_settings! {
+    "assert_unwind_safe" => AssertUnwindSafe,
+    "allow_not_macro"    => AllowNotMacro,
+    "proc_macro_hack"    => ProcMacroHack,
+}
+
+pub(crate) fn parse_settings(input: TokenStream) -> Result<Settings> {
+    let mut input = input.into_iter();
+    let mut res = Settings(Vec::new());
+    loop {
+        match input.next() {
+            Some(TokenTree::Ident(ident)) => {
+                res.0.push(ident_to_setting(ident)?);
+            }
+            None => return Ok(res),
+            other => {
+                let span = other.map_or(Span::call_site(), |tt| tt.span());
+                return Err(Error::new(span, "expected identifier".to_string()));
+            }
+        }
+
+        match input.next() {
+            Some(TokenTree::Punct(ref punct)) if punct.as_char() == ',' => {}
+            None => return Ok(res),
+            other => {
+                let span = other.map_or(Span::call_site(), |tt| tt.span());
+                return Err(Error::new(span, "expected `,`".to_string()));
+            }
+        }
+    }
+}
+
+pub(crate) struct Settings(Vec<Setting>);
+
+impl Settings {
+    pub(crate) fn is_set(&self, setting: Setting) -> bool {
+        self.0.iter().any(|s| *s == setting)
+    }
+
+    pub(crate) fn set(&mut self, setting: Setting) {
+        self.0.push(setting)
+    }
+}