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/nrf52840-pac/src/mwu/perregion.rs

@@ -0,0 +1,16 @@
+#[doc = r"Register block"]
+#[repr(C)]
+pub struct PERREGION {
+    #[doc = "0x00 - Description cluster\\[n\\]: Source of event/interrupt in region n, write access detected while corresponding subregion was enabled for watching"]
+    pub substatwa: SUBSTATWA,
+    #[doc = "0x04 - Description cluster\\[n\\]: Source of event/interrupt in region n, read access detected while corresponding subregion was enabled for watching"]
+    pub substatra: SUBSTATRA,
+}
+#[doc = "SUBSTATWA (rw) register accessor: an alias for `Reg<SUBSTATWA_SPEC>`"]
+pub type SUBSTATWA = crate::Reg<substatwa::SUBSTATWA_SPEC>;
+#[doc = "Description cluster\\[n\\]: Source of event/interrupt in region n, write access detected while corresponding subregion was enabled for watching"]
+pub mod substatwa;
+#[doc = "SUBSTATRA (rw) register accessor: an alias for `Reg<SUBSTATRA_SPEC>`"]
+pub type SUBSTATRA = crate::Reg<substatra::SUBSTATRA_SPEC>;
+#[doc = "Description cluster\\[n\\]: Source of event/interrupt in region n, read access detected while corresponding subregion was enabled for watching"]
+pub mod substatra;