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/minijinja/src/debug.rs

@@ -0,0 +1,90 @@
+use std::collections::BTreeMap;
+use std::fmt;
+
+use crate::compiler::tokens::Span;
+use crate::error::ErrorKind;
+use crate::value::Value;
+
+/// This is a snapshot of the debug information.
+#[cfg_attr(docsrs, doc(cfg(feature = "debug")))]
+#[derive(Default)]
+pub(crate) struct DebugInfo {
+    pub(crate) template_source: Option<String>,
+    pub(crate) referenced_locals: BTreeMap<String, Value>,
+}
+
+struct VarPrinter<'x>(&'x BTreeMap<String, Value>);
+
+impl fmt::Debug for VarPrinter<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if self.0.is_empty() {
+            return f.write_str("No referenced variables");
+        }
+        let mut m = f.debug_struct("Referenced variables:");
+        let mut vars = self.0.iter().collect::<Vec<_>>();
+        vars.sort_by_key(|x| x.0);
+        for (key, value) in vars {
+            m.field(key, value);
+        }
+        m.finish()
+    }
+}
+
+impl DebugInfo {
+    pub fn source(&self) -> Option<&str> {
+        self.template_source.as_deref()
+    }
+}
+
+pub(super) fn render_debug_info(
+    f: &mut fmt::Formatter,
+    name: Option<&str>,
+    kind: ErrorKind,
+    line: Option<usize>,
+    span: Option<Span>,
+    info: &DebugInfo,
+) -> fmt::Result {
+    if let Some(source) = info.source() {
+        let title = format!(
+            " {} ",
+            name.unwrap_or_default()
+                .rsplit(&['/', '\\'])
+                .next()
+                .unwrap_or("Template Source")
+        );
+        ok!(writeln!(f));
+        writeln!(f, "{:-^1$}", title, 79).unwrap();
+        let lines: Vec<_> = source.lines().enumerate().collect();
+        let idx = line.unwrap_or(1).saturating_sub(1);
+        let skip = idx.saturating_sub(3);
+        let pre = lines.iter().skip(skip).take(3.min(idx)).collect::<Vec<_>>();
+        let post = lines.iter().skip(idx + 1).take(3).collect::<Vec<_>>();
+        for (idx, line) in pre {
+            writeln!(f, "{:>4} | {}", idx + 1, line).unwrap();
+        }
+
+        if let Some(line) = lines.get(idx) {
+            writeln!(f, "{:>4} > {}", idx + 1, line.1).unwrap();
+        }
+        if let Some(span) = span {
+            if span.start_line == span.end_line {
+                ok!(writeln!(
+                    f,
+                    "     i {}{} {}",
+                    " ".repeat(span.start_col as usize),
+                    "^".repeat(span.end_col as usize - span.start_col as usize),
+                    kind,
+                ));
+            }
+        }
+
+        for (idx, line) in post {
+            writeln!(f, "{:>4} | {}", idx + 1, line).unwrap();
+        }
+        write!(f, "{:~^1$}", "", 79).unwrap();
+    }
+    ok!(writeln!(f));
+    ok!(writeln!(f, "{:#?}", VarPrinter(&info.referenced_locals)));
+    write!(f, "{:-^1$}", "", 79).unwrap();
+    Ok(())
+}