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/esp-hal/src/i2c/mod.rs

@@ -0,0 +1,11 @@
+//! # Inter-Integrated Circuit (I2C)
+//!
+//! I2C is a serial, synchronous, multi-device, half-duplex communication
+//! protocol that allows co-existence of multiple masters and slaves on the
+//! same bus. I2C uses two bidirectional open-drain lines: serial data line
+//! (SDA) and serial clock line (SCL), pulled up by resistors.
+
+pub mod master;
+
+#[cfg(lp_i2c0)]
+pub mod lp_i2c;