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/once_cell/examples/regex.rs

@@ -0,0 +1,49 @@
+use std::{str::FromStr, time::Instant};
+
+use regex::Regex;
+
+macro_rules! regex {
+    ($re:literal $(,)?) => {{
+        static RE: once_cell::sync::OnceCell<regex::Regex> = once_cell::sync::OnceCell::new();
+        RE.get_or_init(|| regex::Regex::new($re).unwrap())
+    }};
+}
+
+fn slow() {
+    let s = r##"13.28.24.13 - - [10/Mar/2016:19:29:25 +0100] "GET /etc/lib/pChart2/examples/index.php?Action=View&Script=../../../../cnf/db.php HTTP/1.1" 404 151 "-" "HTTP_Request2/2.2.1 (http://pear.php.net/package/http_request2) PHP/5.3.16""##;
+
+    let mut total = 0;
+    for _ in 0..1000 {
+        let re = Regex::new(
+            r##"^(\S+) (\S+) (\S+) \[([^]]+)\] "([^"]*)" (\d+) (\d+) "([^"]*)" "([^"]*)"$"##,
+        )
+        .unwrap();
+        let size = usize::from_str(re.captures(s).unwrap().get(7).unwrap().as_str()).unwrap();
+        total += size;
+    }
+    println!("{}", total);
+}
+
+fn fast() {
+    let s = r##"13.28.24.13 - - [10/Mar/2016:19:29:25 +0100] "GET /etc/lib/pChart2/examples/index.php?Action=View&Script=../../../../cnf/db.php HTTP/1.1" 404 151 "-" "HTTP_Request2/2.2.1 (http://pear.php.net/package/http_request2) PHP/5.3.16""##;
+
+    let mut total = 0;
+    for _ in 0..1000 {
+        let re: &Regex = regex!(
+            r##"^(\S+) (\S+) (\S+) \[([^]]+)\] "([^"]*)" (\d+) (\d+) "([^"]*)" "([^"]*)"$"##,
+        );
+        let size = usize::from_str(re.captures(s).unwrap().get(7).unwrap().as_str()).unwrap();
+        total += size;
+    }
+    println!("{}", total);
+}
+
+fn main() {
+    let t = Instant::now();
+    slow();
+    println!("slow: {:?}", t.elapsed());
+
+    let t = Instant::now();
+    fast();
+    println!("fast: {:?}", t.elapsed());
+}