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/esp32s3/src/nrx.rs

@@ -0,0 +1,18 @@
+#[repr(C)]
+#[cfg_attr(feature = "impl-register-debug", derive(Debug))]
+#[doc = "Register block"]
+pub struct RegisterBlock {
+    _reserved0: [u8; 0xd4],
+    nrxpd_ctrl: NRXPD_CTRL,
+}
+impl RegisterBlock {
+    #[doc = "0xd4 - NRX Power Down Control Register"]
+    #[inline(always)]
+    pub const fn nrxpd_ctrl(&self) -> &NRXPD_CTRL {
+        &self.nrxpd_ctrl
+    }
+}
+#[doc = "NRXPD_CTRL (rw) register accessor: NRX Power Down Control Register\n\nYou can [`read`](crate::Reg::read) this register and get [`nrxpd_ctrl::R`]. You can [`reset`](crate::Reg::reset), [`write`](crate::Reg::write), [`write_with_zero`](crate::Reg::write_with_zero) this register using [`nrxpd_ctrl::W`]. You can also [`modify`](crate::Reg::modify) this register. See [API](https://docs.rs/svd2rust/#read--modify--write-api).\n\nFor information about available fields see [`mod@nrxpd_ctrl`] module"]
+pub type NRXPD_CTRL = crate::Reg<nrxpd_ctrl::NRXPD_CTRL_SPEC>;
+#[doc = "NRX Power Down Control Register"]
+pub mod nrxpd_ctrl;