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/events.rs

@@ -0,0 +1,22 @@
+use std::io::Read;
+
+use pulldown_cmark::{Event, Parser};
+
+/// Show all events from the text on stdin.
+fn main() {
+    let mut text = String::new();
+    std::io::stdin().read_to_string(&mut text).unwrap();
+
+    eprintln!("{text:?} -> [");
+    let mut width = 0;
+    for event in Parser::new(&text) {
+        if let Event::End(_) = event {
+            width -= 2;
+        }
+        eprintln!("  {:width$}{event:?}", "");
+        if let Event::Start(_) = event {
+            width += 2;
+        }
+    }
+    eprintln!("]");
+}