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/riscv/README.md

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+[![crates.io](https://img.shields.io/crates/d/riscv.svg)](https://crates.io/crates/riscv)
+[![crates.io](https://img.shields.io/crates/v/riscv.svg)](https://crates.io/crates/riscv)
+
+# `riscv`
+
+> Low level access to RISC-V processors
+
+This project is developed and maintained by the [RISC-V team][team].
+
+## [Documentation](https://docs.rs/crate/riscv)
+
+## Minimum Supported Rust Version (MSRV)
+
+This crate is guaranteed to compile on stable Rust 1.61 and up. It *might*
+compile with older versions but that may change in any new patch release.
+
+## License
+
+Copyright 2019-2022 [RISC-V team][team]
+
+Permission to use, copy, modify, and/or distribute this software for any purpose
+with or without fee is hereby granted, provided that the above copyright notice
+and this permission notice appear in all copies.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND
+FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS
+OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
+TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF
+THIS SOFTWARE.
+
+## Code of Conduct
+
+Contribution to this crate is organized under the terms of the [Rust Code of
+Conduct][CoC], the maintainer of this crate, the [RISC-V team][team], promises
+to intervene to uphold that code of conduct.
+
+[CoC]: CODE_OF_CONDUCT.md
+[team]: https://github.com/rust-embedded/wg#the-risc-v-team