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/embassy-nrf/src/util.rs

@@ -0,0 +1,23 @@
+#![allow(dead_code)]
+
+const SRAM_LOWER: usize = 0x2000_0000;
+const SRAM_UPPER: usize = 0x3000_0000;
+
+/// Does this slice reside entirely within RAM?
+pub(crate) fn slice_in_ram<T>(slice: *const [T]) -> bool {
+    if slice.is_empty() {
+        return true;
+    }
+
+    let ptr = slice as *const T as usize;
+    ptr >= SRAM_LOWER && (ptr + slice.len() * core::mem::size_of::<T>()) < SRAM_UPPER
+}
+
+/// Return an error if slice is not in RAM. Skips check if slice is zero-length.
+pub(crate) fn slice_in_ram_or<T, E>(slice: *const [T], err: E) -> Result<(), E> {
+    if slice_in_ram(slice) {
+        Ok(())
+    } else {
+        Err(err)
+    }
+}