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/serde_derive/src/fragment.rs

@@ -0,0 +1,74 @@
+use proc_macro2::TokenStream;
+use quote::ToTokens;
+use syn::{token, Token};
+
+pub enum Fragment {
+    /// Tokens that can be used as an expression.
+    Expr(TokenStream),
+    /// Tokens that can be used inside a block. The surrounding curly braces are
+    /// not part of these tokens.
+    Block(TokenStream),
+}
+
+macro_rules! quote_expr {
+    ($($tt:tt)*) => {
+        $crate::fragment::Fragment::Expr(quote!($($tt)*))
+    }
+}
+
+macro_rules! quote_block {
+    ($($tt:tt)*) => {
+        $crate::fragment::Fragment::Block(quote!($($tt)*))
+    }
+}
+
+/// Interpolate a fragment in place of an expression. This involves surrounding
+/// Block fragments in curly braces.
+pub struct Expr(pub Fragment);
+impl ToTokens for Expr {
+    fn to_tokens(&self, out: &mut TokenStream) {
+        match &self.0 {
+            Fragment::Expr(expr) => expr.to_tokens(out),
+            Fragment::Block(block) => {
+                token::Brace::default().surround(out, |out| block.to_tokens(out));
+            }
+        }
+    }
+}
+
+/// Interpolate a fragment as the statements of a block.
+pub struct Stmts(pub Fragment);
+impl ToTokens for Stmts {
+    fn to_tokens(&self, out: &mut TokenStream) {
+        match &self.0 {
+            Fragment::Expr(expr) => expr.to_tokens(out),
+            Fragment::Block(block) => block.to_tokens(out),
+        }
+    }
+}
+
+/// Interpolate a fragment as the value part of a `match` expression. This
+/// involves putting a comma after expressions and curly braces around blocks.
+pub struct Match(pub Fragment);
+impl ToTokens for Match {
+    fn to_tokens(&self, out: &mut TokenStream) {
+        match &self.0 {
+            Fragment::Expr(expr) => {
+                expr.to_tokens(out);
+                <Token![,]>::default().to_tokens(out);
+            }
+            Fragment::Block(block) => {
+                token::Brace::default().surround(out, |out| block.to_tokens(out));
+            }
+        }
+    }
+}
+
+impl AsRef<TokenStream> for Fragment {
+    fn as_ref(&self) -> &TokenStream {
+        match self {
+            Fragment::Expr(expr) => expr,
+            Fragment::Block(block) => block,
+        }
+    }
+}