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/mwu/events_pregion.rs

@@ -0,0 +1,16 @@
+#[doc = r"Register block"]
+#[repr(C)]
+pub struct EVENTS_PREGION {
+    #[doc = "0x00 - Description cluster\\[0\\]: Write access to peripheral region 0 detected"]
+    pub wa: WA,
+    #[doc = "0x04 - Description cluster\\[0\\]: Read access to peripheral region 0 detected"]
+    pub ra: RA,
+}
+#[doc = "WA (rw) register accessor: an alias for `Reg<WA_SPEC>`"]
+pub type WA = crate::Reg<wa::WA_SPEC>;
+#[doc = "Description cluster\\[0\\]: Write access to peripheral region 0 detected"]
+pub mod wa;
+#[doc = "RA (rw) register accessor: an alias for `Reg<RA_SPEC>`"]
+pub type RA = crate::Reg<ra::RA_SPEC>;
+#[doc = "Description cluster\\[0\\]: Read access to peripheral region 0 detected"]
+pub mod ra;