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/ficr/info.rs

@@ -0,0 +1,40 @@
+#[doc = r"Register block"]
+#[repr(C)]
+pub struct INFO {
+    #[doc = "0x00 - Part code"]
+    pub part: PART,
+    #[doc = "0x04 - Build code (hardware version and production configuration)"]
+    pub variant: VARIANT,
+    #[doc = "0x08 - Package option"]
+    pub package: PACKAGE,
+    #[doc = "0x0c - RAM variant"]
+    pub ram: RAM,
+    #[doc = "0x10 - Flash variant"]
+    pub flash: FLASH,
+    #[doc = "0x14..0x20 - Unspecified"]
+    pub unused8: [UNUSED8; 3],
+}
+#[doc = "PART (r) register accessor: an alias for `Reg<PART_SPEC>`"]
+pub type PART = crate::Reg<part::PART_SPEC>;
+#[doc = "Part code"]
+pub mod part;
+#[doc = "VARIANT (r) register accessor: an alias for `Reg<VARIANT_SPEC>`"]
+pub type VARIANT = crate::Reg<variant::VARIANT_SPEC>;
+#[doc = "Build code (hardware version and production configuration)"]
+pub mod variant;
+#[doc = "PACKAGE (r) register accessor: an alias for `Reg<PACKAGE_SPEC>`"]
+pub type PACKAGE = crate::Reg<package::PACKAGE_SPEC>;
+#[doc = "Package option"]
+pub mod package;
+#[doc = "RAM (r) register accessor: an alias for `Reg<RAM_SPEC>`"]
+pub type RAM = crate::Reg<ram::RAM_SPEC>;
+#[doc = "RAM variant"]
+pub mod ram;
+#[doc = "FLASH (r) register accessor: an alias for `Reg<FLASH_SPEC>`"]
+pub type FLASH = crate::Reg<flash::FLASH_SPEC>;
+#[doc = "Flash variant"]
+pub mod flash;
+#[doc = "UNUSED8 (rw) register accessor: an alias for `Reg<UNUSED8_SPEC>`"]
+pub type UNUSED8 = crate::Reg<unused8::UNUSED8_SPEC>;
+#[doc = "Unspecified"]
+pub mod unused8;