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/vcell/README.md

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+[![crates.io](https://img.shields.io/crates/v/vcell.svg)](https://crates.io/crates/vcell)
+[![crates.io](https://img.shields.io/crates/d/vcell.svg)](https://crates.io/crates/vcell)
+
+# `vcell`
+
+> Just like [`Cell`] but with [volatile] read / write operations
+
+[`Cell`]: https://doc.rust-lang.org/std/cell/struct.Cell.html
+[volatile]: https://doc.rust-lang.org/std/ptr/fn.read_volatile.html
+
+# [Change log](CHANGELOG.md)
+
+# License
+
+Licensed under either of
+
+- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
+  http://www.apache.org/licenses/LICENSE-2.0)
+- MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+
+at your option.
+
+## Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.