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-riscv-rt/README.md

@@ -0,0 +1,35 @@
+# esp-riscv-rt
+
+[![Crates.io](https://img.shields.io/crates/v/esp-riscv-rt?labelColor=1C2C2E&color=C96329&logo=Rust&style=flat-square)](https://crates.io/crates/esp-riscv-rt)
+[![docs.rs](https://img.shields.io/docsrs/esp-riscv-rt?labelColor=1C2C2E&color=C96329&logo=rust&style=flat-square)](https://docs.rs/esp-riscv-rt)
+![MSRV](https://img.shields.io/badge/MSRV-1.60-blue?labelColor=1C2C2E&style=flat-square)
+![Crates.io](https://img.shields.io/crates/l/esp-riscv-rt?labelColor=1C2C2E&style=flat-square)
+[![Matrix](https://img.shields.io/matrix/esp-rs:matrix.org?label=join%20matrix&labelColor=1C2C2E&color=BEC5C9&logo=matrix&style=flat-square)](https://matrix.to/#/#esp-rs:matrix.org)
+
+Minimal runtime / startup for RISC-V devices from Espressif.
+
+Much of the code in this package originated in the [rust-embedded/riscv] repository.
+
+[rust-embedded/riscv]: https://github.com/rust-embedded/riscv
+
+## [Documentation](https://docs.rs/crate/esp-riscv-rt)
+
+## Minimum Supported Rust Version (MSRV)
+
+This crate is guaranteed to compile on stable Rust 1.65 and up. It _might_
+compile with older versions but that may change in any new patch release.
+
+## 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.