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/cobs/CHANGELOG.md

@@ -0,0 +1,47 @@
+Change Log
+=======
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](http://keepachangelog.com/)
+and this project adheres to [Semantic Versioning](http://semver.org/).
+
+# [unreleased]
+
+# [v0.3.0] 2025-01-31
+
+## Added
+
+- Added `max_encoding_overhead` const function.
+  [#20](https://github.com/jamesmunns/cobs.rs/pull/20)
+- Unittests for `decode_in_place` and `decode_in_place_report`.
+  [#36](https://github.com/jamesmunns/cobs.rs/pull/36)
+- New `alloc` feature which enables support for APIs returning `Vec`s.
+  [#24](https://github.com/jamesmunns/cobs.rs/pull/24)
+- New `std` feature, deprecate the `use_std` feature but keep it for backwards compatiblity.
+  [#38](https://github.com/jamesmunns/cobs.rs/pull/38)
+- Documentation of the new `std` and `alloc` features in the README.
+  [#38](https://github.com/jamesmunns/cobs.rs/pull/38)
+- Optional `defmt` support feature-gated by the optional `defmt` feature.
+  [#40](https://github.com/jamesmunns/cobs.rs/pull/40)
+- Add optional `serde` support that adds `serde` derives for some errors and data structures.
+  [#41](https://github.com/jamesmunns/cobs.rs/pull/41)
+
+## Changed
+
+- Bumped rust edition to 2021
+- Improved error handling by replacing unit errors with a new `DecodeError` enumeration for
+  the decoding module and a `DestBufTooSmallError` for the encoding module.
+  [#30](https://github.com/jamesmunns/cobs.rs/pull/30)
+- `max_encoding_length` is now a const function. [#20](https://github.com/jamesmunns/cobs.rs/pull/20)
+- `decode_in_place` and `decode_in_place_report` now check the destination index and return
+  an appropriate `DecodeError` instead of panicking if an out-of-bounds access happens.
+  [#36](https://github.com/jamesmunns/cobs.rs/pull/36)
+- Consistent behaviour of decode functions for empty input: All funtions now return
+  `DecodeError::FrameEmpty`.
+  [#39](https://github.com/jamesmunns/cobs.rs/pull/39)
+
+## Fixed
+
+- Fixed wrong encoded length when source length was divisible by 254.
+  [#19](https://github.com/jamesmunns/cobs.rs/issues/19)