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/src/fnv.rs

@@ -0,0 +1,37 @@
+use crate::Hasher as _;
+
+const BASIS: u32 = 0x811c9dc5;
+const PRIME: u32 = 0x1000193;
+
+/// 32-bit Fowler-Noll-Vo hasher
+pub struct Hasher {
+    state: u32,
+}
+
+impl Default for Hasher {
+    fn default() -> Self {
+        Hasher { state: BASIS }
+    }
+}
+
+impl crate::Hasher for Hasher {
+    #[inline]
+    fn finish32(&self) -> u32 {
+        self.state
+    }
+}
+
+impl core::hash::Hasher for Hasher {
+    #[inline]
+    fn write(&mut self, bytes: &[u8]) {
+        for byte in bytes {
+            self.state ^= u32::from(*byte);
+            self.state = self.state.wrapping_mul(PRIME);
+        }
+    }
+
+    #[inline]
+    fn finish(&self) -> u64 {
+        self.finish32().into()
+    }
+}