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/rustversion/src/token.rs

@@ -0,0 +1,78 @@
+use crate::error::{Error, Result};
+use crate::iter::Iter;
+use proc_macro::{Delimiter, Group, Ident, Literal, Span, TokenTree};
+
+pub fn parse_punct(iter: Iter, ch: char) -> Result<()> {
+    match iter.next() {
+        Some(TokenTree::Punct(ref punct)) if punct.as_char() == ch => Ok(()),
+        unexpected => {
+            let span = unexpected
+                .as_ref()
+                .map_or_else(Span::call_site, TokenTree::span);
+            Err(Error::new(span, format!("expected `{}`", ch)))
+        }
+    }
+}
+
+pub fn parse_optional_punct(iter: Iter, ch: char) -> Option<()> {
+    match iter.peek() {
+        Some(TokenTree::Punct(punct)) if punct.as_char() == ch => iter.next().map(drop),
+        _ => None,
+    }
+}
+
+pub fn parse_optional_keyword(iter: Iter, keyword: &str) -> Option<Span> {
+    match iter.peek() {
+        Some(TokenTree::Ident(ident)) if ident.to_string() == keyword => {
+            Some(iter.next().unwrap().span())
+        }
+        _ => None,
+    }
+}
+
+pub fn parse_literal(iter: Iter) -> Result<Literal> {
+    match iter.next() {
+        Some(TokenTree::Literal(literal)) => Ok(literal),
+        unexpected => {
+            let span = unexpected
+                .as_ref()
+                .map_or_else(Span::call_site, TokenTree::span);
+            Err(Error::new(span, "expected literal"))
+        }
+    }
+}
+
+pub fn parse_paren(introducer: &Ident, iter: Iter) -> Result<Group> {
+    match iter.peek() {
+        Some(TokenTree::Group(group)) if group.delimiter() == Delimiter::Parenthesis => {
+            match iter.next() {
+                Some(TokenTree::Group(group)) => Ok(group),
+                _ => unreachable!(),
+            }
+        }
+        Some(unexpected) => Err(Error::new(unexpected.span(), "expected `(`")),
+        None => Err(Error::new(
+            introducer.span(),
+            format!("expected `(` after `{}`", introducer),
+        )),
+    }
+}
+
+pub fn parse_optional_paren(iter: Iter) -> Option<Group> {
+    match iter.peek() {
+        Some(TokenTree::Group(group)) if group.delimiter() == Delimiter::Parenthesis => {
+            match iter.next() {
+                Some(TokenTree::Group(group)) => Some(group),
+                _ => unreachable!(),
+            }
+        }
+        _ => None,
+    }
+}
+
+pub fn parse_end(iter: Iter) -> Result<()> {
+    match iter.next() {
+        None => Ok(()),
+        Some(unexpected) => Err(Error::new(unexpected.span(), "unexpected token")),
+    }
+}