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/spi0.rs

@@ -0,0 +1,66 @@
+#[doc = r"Register block"]
+#[repr(C)]
+pub struct RegisterBlock {
+    _reserved0: [u8; 0x0108],
+    #[doc = "0x108 - TXD byte sent and RXD byte received"]
+    pub events_ready: EVENTS_READY,
+    _reserved1: [u8; 0x01f8],
+    #[doc = "0x304 - Enable interrupt"]
+    pub intenset: INTENSET,
+    #[doc = "0x308 - Disable interrupt"]
+    pub intenclr: INTENCLR,
+    _reserved3: [u8; 0x01f4],
+    #[doc = "0x500 - Enable SPI"]
+    pub enable: ENABLE,
+    _reserved4: [u8; 0x04],
+    #[doc = "0x508..0x514 - Unspecified"]
+    pub psel: PSEL,
+    _reserved5: [u8; 0x04],
+    #[doc = "0x518 - RXD register"]
+    pub rxd: RXD,
+    #[doc = "0x51c - TXD register"]
+    pub txd: TXD,
+    _reserved7: [u8; 0x04],
+    #[doc = "0x524 - SPI frequency. Accuracy depends on the HFCLK source selected."]
+    pub frequency: FREQUENCY,
+    _reserved8: [u8; 0x2c],
+    #[doc = "0x554 - Configuration register"]
+    pub config: CONFIG,
+}
+#[doc = "EVENTS_READY (rw) register accessor: an alias for `Reg<EVENTS_READY_SPEC>`"]
+pub type EVENTS_READY = crate::Reg<events_ready::EVENTS_READY_SPEC>;
+#[doc = "TXD byte sent and RXD byte received"]
+pub mod events_ready;
+#[doc = "INTENSET (rw) register accessor: an alias for `Reg<INTENSET_SPEC>`"]
+pub type INTENSET = crate::Reg<intenset::INTENSET_SPEC>;
+#[doc = "Enable interrupt"]
+pub mod intenset;
+#[doc = "INTENCLR (rw) register accessor: an alias for `Reg<INTENCLR_SPEC>`"]
+pub type INTENCLR = crate::Reg<intenclr::INTENCLR_SPEC>;
+#[doc = "Disable interrupt"]
+pub mod intenclr;
+#[doc = "ENABLE (rw) register accessor: an alias for `Reg<ENABLE_SPEC>`"]
+pub type ENABLE = crate::Reg<enable::ENABLE_SPEC>;
+#[doc = "Enable SPI"]
+pub mod enable;
+#[doc = "Unspecified"]
+pub use psel::PSEL;
+#[doc = r"Cluster"]
+#[doc = "Unspecified"]
+pub mod psel;
+#[doc = "RXD (r) register accessor: an alias for `Reg<RXD_SPEC>`"]
+pub type RXD = crate::Reg<rxd::RXD_SPEC>;
+#[doc = "RXD register"]
+pub mod rxd;
+#[doc = "TXD (rw) register accessor: an alias for `Reg<TXD_SPEC>`"]
+pub type TXD = crate::Reg<txd::TXD_SPEC>;
+#[doc = "TXD register"]
+pub mod txd;
+#[doc = "FREQUENCY (rw) register accessor: an alias for `Reg<FREQUENCY_SPEC>`"]
+pub type FREQUENCY = crate::Reg<frequency::FREQUENCY_SPEC>;
+#[doc = "SPI frequency. Accuracy depends on the HFCLK source selected."]
+pub mod frequency;
+#[doc = "CONFIG (rw) register accessor: an alias for `Reg<CONFIG_SPEC>`"]
+pub type CONFIG = crate::Reg<config::CONFIG_SPEC>;
+#[doc = "Configuration register"]
+pub mod config;