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/winnow/examples/json/main.rs

@@ -0,0 +1,94 @@
+mod json;
+mod parser_alt;
+mod parser_dispatch;
+#[allow(dead_code)]
+mod parser_partial;
+
+use winnow::error::EmptyError;
+use winnow::prelude::*;
+
+fn main() -> Result<(), lexopt::Error> {
+    let args = Args::parse()?;
+
+    let data = args.input.as_deref().unwrap_or(if args.invalid {
+        "  { \"a\"\t: 42,
+  \"b\": [ \"x\", \"y\", 12 ] ,
+  \"c\": { 1\"hello\" : \"world\"
+  }
+  } "
+    } else {
+        "  { \"a\"\t: 42,
+  \"b\": [ \"x\", \"y\", 12 ] ,
+  \"c\": { \"hello\" : \"world\"
+  }
+  } "
+    });
+
+    let result = match args.implementation {
+        Impl::Naive => parser_alt::json::<EmptyError>.parse(data),
+        Impl::Dispatch => parser_dispatch::json::<EmptyError>.parse(data),
+    };
+    match result {
+        Ok(json) => {
+            println!("{json:#?}");
+        }
+        Err(err) => {
+            if args.pretty {
+                println!("{err}");
+            } else {
+                println!("{err:#?}");
+            }
+        }
+    }
+
+    Ok(())
+}
+
+#[derive(Default)]
+struct Args {
+    input: Option<String>,
+    invalid: bool,
+    pretty: bool,
+    implementation: Impl,
+}
+
+#[derive(Default)]
+enum Impl {
+    #[default]
+    Naive,
+    Dispatch,
+}
+
+impl Args {
+    fn parse() -> Result<Self, lexopt::Error> {
+        use lexopt::prelude::*;
+
+        let mut res = Args::default();
+
+        let mut args = lexopt::Parser::from_env();
+        while let Some(arg) = args.next()? {
+            match arg {
+                Long("invalid") => {
+                    res.invalid = true;
+                }
+                Long("pretty") => {
+                    // Only case where pretty matters
+                    res.pretty = true;
+                    res.invalid = true;
+                }
+                Long("impl") => {
+                    res.implementation = args.value()?.parse_with(|s| match s {
+                        "naive" => Ok(Impl::Naive),
+                        "dispatch" => Ok(Impl::Dispatch),
+                        _ => Err("expected `naive`, `dispatch`"),
+                    })?;
+                }
+                Value(input) => {
+                    res.input = Some(input.string()?);
+                }
+                _ => return Err(arg.unexpected()),
+            }
+        }
+        Ok(res)
+    }
+}