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/nrf52832-pac/src/i2s/rxd.rs

@@ -0,0 +1,10 @@
+#[doc = r"Register block"]
+#[repr(C)]
+pub struct RXD {
+    #[doc = "0x00 - Receive buffer RAM start address."]
+    pub ptr: PTR,
+}
+#[doc = "PTR (rw) register accessor: an alias for `Reg<PTR_SPEC>`"]
+pub type PTR = crate::Reg<ptr::PTR_SPEC>;
+#[doc = "Receive buffer RAM start address."]
+pub mod ptr;