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/nrf9120-pac/src/spu_s/dppi.rs

@@ -0,0 +1,16 @@
+#[doc = r"Register block"]
+#[repr(C)]
+pub struct DPPI {
+    #[doc = "0x00 - Description cluster: Select between secure and non-secure attribute for the DPPI channels."]
+    pub perm: PERM,
+    #[doc = "0x04 - Description cluster: Prevent further modification of the corresponding PERM register"]
+    pub lock: LOCK,
+}
+#[doc = "PERM (rw) register accessor: an alias for `Reg<PERM_SPEC>`"]
+pub type PERM = crate::Reg<perm::PERM_SPEC>;
+#[doc = "Description cluster: Select between secure and non-secure attribute for the DPPI channels."]
+pub mod perm;
+#[doc = "LOCK (rw) register accessor: an alias for `Reg<LOCK_SPEC>`"]
+pub type LOCK = crate::Reg<lock::LOCK_SPEC>;
+#[doc = "Description cluster: Prevent further modification of the corresponding PERM register"]
+pub mod lock;