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/pulldown-cmark/examples/parser-map-event-print.rs

@@ -0,0 +1,36 @@
+use pulldown_cmark::{html, Event, Parser};
+
+fn main() {
+    let markdown_input = "# Example Heading\nExample paragraph with **lorem** _ipsum_ text.";
+    println!(
+        "\nParsing the following markdown string:\n{}\n",
+        markdown_input
+    );
+
+    // Set up the parser. We can treat is as any other iterator.
+    // For each event, we print its details, such as the tag or string.
+    // This filter simply returns the same event without any changes;
+    // you can compare the `event-filter` example which alters the output.
+    let parser = Parser::new(markdown_input).map(|event| {
+        match &event {
+            Event::Start(tag) => println!("Start: {:?}", tag),
+            Event::End(tag) => println!("End: {:?}", tag),
+            Event::Html(s) => println!("Html: {:?}", s),
+            Event::InlineHtml(s) => println!("InlineHtml: {:?}", s),
+            Event::Text(s) => println!("Text: {:?}", s),
+            Event::Code(s) => println!("Code: {:?}", s),
+            Event::DisplayMath(s) => println!("DisplayMath: {:?}", s),
+            Event::InlineMath(s) => println!("Math: {:?}", s),
+            Event::FootnoteReference(s) => println!("FootnoteReference: {:?}", s),
+            Event::TaskListMarker(b) => println!("TaskListMarker: {:?}", b),
+            Event::SoftBreak => println!("SoftBreak"),
+            Event::HardBreak => println!("HardBreak"),
+            Event::Rule => println!("Rule"),
+        };
+        event
+    });
+
+    let mut html_output = String::new();
+    html::push_html(&mut html_output, parser);
+    println!("\nHTML output:\n{}\n", &html_output);
+}