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/tests/lib.rs

@@ -0,0 +1,52 @@
+#![cfg(feature = "html")]
+
+use pulldown_cmark::{Options, Parser};
+
+mod suite;
+
+#[inline(never)]
+pub fn test_markdown_html(
+    input: &str,
+    output: &str,
+    smart_punct: bool,
+    metadata_blocks: bool,
+    old_footnotes: bool,
+) {
+    let mut s = String::new();
+
+    let mut opts = Options::empty();
+    opts.insert(Options::ENABLE_MATH);
+    opts.insert(Options::ENABLE_TABLES);
+    opts.insert(Options::ENABLE_STRIKETHROUGH);
+    opts.insert(Options::ENABLE_TASKLISTS);
+    opts.insert(Options::ENABLE_GFM);
+    if old_footnotes {
+        opts.insert(Options::ENABLE_OLD_FOOTNOTES);
+    } else {
+        opts.insert(Options::ENABLE_FOOTNOTES);
+    }
+    if metadata_blocks {
+        opts.insert(Options::ENABLE_YAML_STYLE_METADATA_BLOCKS);
+        opts.insert(Options::ENABLE_PLUSES_DELIMITED_METADATA_BLOCKS);
+    }
+    if smart_punct {
+        opts.insert(Options::ENABLE_SMART_PUNCTUATION);
+    }
+    opts.insert(Options::ENABLE_HEADING_ATTRIBUTES);
+
+    let p = Parser::new_ext(input, opts);
+    pulldown_cmark::html::push_html(&mut s, p);
+
+    // normalizing the HTML using html5ever may hide actual errors
+    // assert_eq!(html_standardize(output), html_standardize(&s));
+    assert_eq!(html_standardize(output), html_standardize(&s));
+}
+
+fn html_standardize(s: &str) -> String {
+    s.replace("<br>", "<br />")
+        .replace("<br/>", "<br />")
+        .replace("<hr>", "<hr />")
+        .replace("<hr/>", "<hr />")
+        // permit extra or missing line breaks only between tags
+        .replace(">\n<", "><")
+}