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/spis0_ns/txd.rs

@@ -0,0 +1,28 @@
+#[doc = r"Register block"]
+#[repr(C)]
+pub struct TXD {
+    #[doc = "0x00 - TXD data pointer"]
+    pub ptr: PTR,
+    #[doc = "0x04 - Maximum number of bytes in transmit buffer"]
+    pub maxcnt: MAXCNT,
+    #[doc = "0x08 - Number of bytes transmitted in last granted transaction"]
+    pub amount: AMOUNT,
+    #[doc = "0x0c - EasyDMA list type"]
+    pub list: LIST,
+}
+#[doc = "PTR (rw) register accessor: an alias for `Reg<PTR_SPEC>`"]
+pub type PTR = crate::Reg<ptr::PTR_SPEC>;
+#[doc = "TXD data pointer"]
+pub mod ptr;
+#[doc = "MAXCNT (rw) register accessor: an alias for `Reg<MAXCNT_SPEC>`"]
+pub type MAXCNT = crate::Reg<maxcnt::MAXCNT_SPEC>;
+#[doc = "Maximum number of bytes in transmit buffer"]
+pub mod maxcnt;
+#[doc = "AMOUNT (r) register accessor: an alias for `Reg<AMOUNT_SPEC>`"]
+pub type AMOUNT = crate::Reg<amount::AMOUNT_SPEC>;
+#[doc = "Number of bytes transmitted in last granted transaction"]
+pub mod amount;
+#[doc = "LIST (rw) register accessor: an alias for `Reg<LIST_SPEC>`"]
+pub type LIST = crate::Reg<list::LIST_SPEC>;
+#[doc = "EasyDMA list type"]
+pub mod list;