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/memo-map/README.md

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+# memo-map
+
+[![Build Status](https://github.com/mitsuhiko/memo-map/workflows/Tests/badge.svg?branch=main)](https://github.com/mitsuhiko/memo-map/actions?query=workflow%3ATests)
+[![Crates.io](https://img.shields.io/crates/d/memo-map.svg)](https://crates.io/crates/memo-map)
+[![License](https://img.shields.io/github/license/mitsuhiko/memo-map)](https://github.com/mitsuhiko/memo-map/blob/main/LICENSE)
+[![rustc 1.41.0](https://img.shields.io/badge/rust-1.43%2B-orange.svg)](https://img.shields.io/badge/rust-1.43%2B-orange.svg)
+[![Documentation](https://docs.rs/memo-map/badge.svg)](https://docs.rs/memo-map)
+
+A concurrent insert only hash map.
+
+This crate implements a “memo map” which is in many ways similar to a HashMap with some crucial differences:
+
+* Unlike a regular hash map, a memo map is thread safe and synchronized.
+* Adding or retrieving keys works through a shared reference, removing only
+  through a mutable reference.
+* Retrieving a value from a memo map returns a plain old reference.
+
+```rust
+use memo_map::MemoMap;
+
+let memo = MemoMap::new();
+let one = memo.get_or_insert(&1, || "one".to_string());
+let one2 = memo.get_or_insert(&1, || "not one".to_string());
+assert_eq!(one, "one");
+assert_eq!(one2, "one");
+```
+
+## License and Links
+
+- [Documentation](https://docs.rs/memo-map/)
+- [Issue Tracker](https://github.com/mitsuhiko/memo-map/issues)
+- License: [Apache-2.0](https://github.com/mitsuhiko/memo-map/blob/main/LICENSE)
+