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/pwm0/seq.rs

@@ -0,0 +1,28 @@
+#[doc = r"Register block"]
+#[repr(C)]
+pub struct SEQ {
+    #[doc = "0x00 - Description cluster\\[n\\]: Beginning address in RAM of this sequence"]
+    pub ptr: PTR,
+    #[doc = "0x04 - Description cluster\\[n\\]: Number of values (duty cycles) in this sequence"]
+    pub cnt: CNT,
+    #[doc = "0x08 - Description cluster\\[n\\]: Number of additional PWM periods between samples loaded into compare register"]
+    pub refresh: REFRESH,
+    #[doc = "0x0c - Description cluster\\[n\\]: Time added after the sequence"]
+    pub enddelay: ENDDELAY,
+}
+#[doc = "PTR (rw) register accessor: an alias for `Reg<PTR_SPEC>`"]
+pub type PTR = crate::Reg<ptr::PTR_SPEC>;
+#[doc = "Description cluster\\[n\\]: Beginning address in RAM of this sequence"]
+pub mod ptr;
+#[doc = "CNT (rw) register accessor: an alias for `Reg<CNT_SPEC>`"]
+pub type CNT = crate::Reg<cnt::CNT_SPEC>;
+#[doc = "Description cluster\\[n\\]: Number of values (duty cycles) in this sequence"]
+pub mod cnt;
+#[doc = "REFRESH (rw) register accessor: an alias for `Reg<REFRESH_SPEC>`"]
+pub type REFRESH = crate::Reg<refresh::REFRESH_SPEC>;
+#[doc = "Description cluster\\[n\\]: Number of additional PWM periods between samples loaded into compare register"]
+pub mod refresh;
+#[doc = "ENDDELAY (rw) register accessor: an alias for `Reg<ENDDELAY_SPEC>`"]
+pub type ENDDELAY = crate::Reg<enddelay::ENDDELAY_SPEC>;
+#[doc = "Description cluster\\[n\\]: Time added after the sequence"]
+pub mod enddelay;