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/broken-link-callbacks.rs

@@ -0,0 +1,35 @@
+use pulldown_cmark::{html, BrokenLink, Options, Parser};
+
+fn main() {
+    let input: &str = "Hello world, check out [my website][].";
+    println!("Parsing the following markdown string:\n{}", input);
+
+    // Setup callback that sets the URL and title when it encounters
+    // a reference to our home page.
+    let callback = |broken_link: BrokenLink| {
+        if broken_link.reference.as_ref() == "my website" {
+            println!(
+                "Replacing the markdown `{}` of type {:?} with a working link",
+                &input[broken_link.span], broken_link.link_type,
+            );
+            Some(("http://example.com".into(), "my example website".into()))
+        } else {
+            None
+        }
+    };
+
+    // Create a parser with our callback function for broken links.
+    let parser = Parser::new_with_broken_link_callback(input, Options::empty(), Some(callback));
+
+    // Write to String buffer.
+    let mut html_output: String = String::with_capacity(input.len() * 3 / 2);
+    html::push_html(&mut html_output, parser);
+
+    // Check that the output is what we expected.
+    let expected_html: &str =
+        "<p>Hello world, check out <a href=\"http://example.com\" title=\"my example website\">my website</a>.</p>\n";
+    assert_eq!(expected_html, &html_output);
+
+    // Write result to stdout.
+    println!("\nHTML output:\n{}", &html_output);
+}