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/lora-modulation/README.md

@@ -0,0 +1,43 @@
+# lora-modulation
+
+[![Latest Version]][crates.io]
+[![Docs]][doc.rs]
+
+A minimal crate for providing LoRa modulation characteristics of:
+* Bandwidth
+* Spreading factor
+* Coding rate
+
+Provides utility for calculating time on air.
+
+## Usage
+
+```rust
+use lora_modulation::{BaseBandModulationParams, SpreadingFactor, Bandwidth, CodingRate};
+
+let length = 12;
+let params = BaseBandModulationParams::new(SpreadingFactor::_9, Bandwidth::_125KHz, CodingRate::_4_5);
+let time_on_air = params.time_on_air_us(
+    Some(8), // preamble
+    true,    // explicit header
+    length); // length of payload
+
+// Time on air is 144.384 ms
+assert_eq!(time_on_air, 144384);
+```
+
+```rust
+use lora_modulation::{BaseBandModulationParams, SpreadingFactor, Bandwidth, CodingRate};
+
+let symbols = 14;
+let params = BaseBandModulationParams::new(SpreadingFactor::_12, Bandwidth::_125KHz, CodingRate::_4_5);
+let timeout = params.symbols_to_ms(symbols);
+
+// Timeout is 458 ms
+assert_eq!(timeout, 458);
+```
+
+[Latest Version]: https://img.shields.io/crates/v/lora-modulation.svg
+[crates.io]: https://crates.io/crates/lora-modulation
+[Docs]: https://docs.rs/lora-modulation/badge.svg
+[doc.rs]: https://docs.rs/lora-modulation