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/rustc_version/.travis.yml

@@ -0,0 +1,21 @@
+language: rust
+rust:
+- nightly
+- beta
+- stable
+before_script:
+- |
+  pip install 'travis-cargo<0.2' --user &&
+  export PATH=$HOME/.local/bin:$PATH
+script:
+- |
+  travis-cargo build &&
+  travis-cargo test &&
+  travis-cargo bench &&
+  travis-cargo --only stable doc
+after_success:
+- travis-cargo --only stable doc-upload
+env:
+  global:
+  - TRAVIS_CARGO_NIGHTLY_FEATURE=""
+  - secure: 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