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/constfn.rs

@@ -0,0 +1,58 @@
+use crate::error::{Error, Result};
+use proc_macro::{Ident, Span, TokenStream, TokenTree};
+use std::iter;
+
+#[derive(PartialOrd, PartialEq)]
+enum Qualifiers {
+    None,
+    Async,
+    Unsafe,
+    Extern,
+    Abi,
+}
+
+impl Qualifiers {
+    fn from_ident(ident: &Ident) -> Self {
+        match ident.to_string().as_str() {
+            "async" => Qualifiers::Async,
+            "unsafe" => Qualifiers::Unsafe,
+            "extern" => Qualifiers::Extern,
+            _ => Qualifiers::None,
+        }
+    }
+}
+
+pub(crate) fn insert_const(input: TokenStream, const_span: Span) -> Result<TokenStream> {
+    let ref mut input = crate::iter::new(input);
+    let mut out = TokenStream::new();
+    let mut qualifiers = Qualifiers::None;
+    let mut pending = Vec::new();
+
+    while let Some(token) = input.next() {
+        match token {
+            TokenTree::Ident(ref ident) if ident.to_string() == "fn" => {
+                let const_ident = Ident::new("const", const_span);
+                out.extend(iter::once(TokenTree::Ident(const_ident)));
+                out.extend(pending);
+                out.extend(iter::once(token));
+                out.extend(input);
+                return Ok(out);
+            }
+            TokenTree::Ident(ref ident) if Qualifiers::from_ident(ident) > qualifiers => {
+                qualifiers = Qualifiers::from_ident(ident);
+                pending.push(token);
+            }
+            TokenTree::Literal(_) if qualifiers == Qualifiers::Extern => {
+                qualifiers = Qualifiers::Abi;
+                pending.push(token);
+            }
+            _ => {
+                qualifiers = Qualifiers::None;
+                out.extend(pending.drain(..));
+                out.extend(iter::once(token));
+            }
+        }
+    }
+
+    Err(Error::new(const_span, "only allowed on a fn item"))
+}