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/hash32/examples/derive.rs

@@ -0,0 +1,26 @@
+extern crate hash32;
+
+use std::hash::Hash;
+
+use hash32::{FnvHasher, Hasher};
+
+#[derive(Hash)]
+struct Led {
+    state: bool,
+}
+
+#[derive(Hash)]
+struct Ipv4Addr([u8; 4]);
+
+#[derive(Hash)]
+struct Generic<T> {
+    inner: T,
+}
+
+fn main() {
+    let mut fnv = FnvHasher::default();
+    Led { state: true }.hash(&mut fnv);
+    Generic { inner: 0 }.hash(&mut fnv);
+    Ipv4Addr([127, 0, 0, 1]).hash(&mut fnv);
+    println!("{}", fnv.finish32())
+}