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/nrf9160-pac/src/vmc_ns.rs

@@ -0,0 +1,33 @@
+#[doc = r"Register block"]
+#[repr(C)]
+pub struct RegisterBlock {
+    _reserved0: [u8; 0x0600],
+    #[doc = "0x600..0x60c - Unspecified"]
+    pub ram0: RAM,
+    _reserved1: [u8; 0x04],
+    #[doc = "0x610..0x61c - Unspecified"]
+    pub ram1: RAM,
+    _reserved2: [u8; 0x04],
+    #[doc = "0x620..0x62c - Unspecified"]
+    pub ram2: RAM,
+    _reserved3: [u8; 0x04],
+    #[doc = "0x630..0x63c - Unspecified"]
+    pub ram3: RAM,
+    _reserved4: [u8; 0x04],
+    #[doc = "0x640..0x64c - Unspecified"]
+    pub ram4: RAM,
+    _reserved5: [u8; 0x04],
+    #[doc = "0x650..0x65c - Unspecified"]
+    pub ram5: RAM,
+    _reserved6: [u8; 0x04],
+    #[doc = "0x660..0x66c - Unspecified"]
+    pub ram6: RAM,
+    _reserved7: [u8; 0x04],
+    #[doc = "0x670..0x67c - Unspecified"]
+    pub ram7: RAM,
+}
+#[doc = "Unspecified"]
+pub use ram::RAM;
+#[doc = r"Cluster"]
+#[doc = "Unspecified"]
+pub mod ram;