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/void/.travis.yml

@@ -0,0 +1,26 @@
+language: rust
+sudo: false
+
+rust:
+    - 1.0.0
+    - stable
+    - beta
+    - nightly
+
+script:
+    - cargo build
+    - cargo test
+    - if [ "$TRAVIS_RUST_VERSION" != "1.0.0" ]; then cargo test --no-default-features; fi
+    - cargo bench --no-run
+    - cargo doc
+
+after_success:
+    - if [ "$TRAVIS_PULL_REQUEST" == false && test == "TRAVIS_BRANCH" == "master" ]; then
+    - curl https://raw.githubusercontent.com/reem/rust-gh-docs/master/make-docs.sh > docs.sh
+    - chmod u+x docs.sh
+    - ./docs.sh reem project-name
+
+env:
+  global:
+    secure: QPYL1XUr4CyK/2DXlsYC1eCpWRpyEiqQSd/FFVR+YdP/rOJ7AyAXQqPhfgjDBQwvc6E2fUiyYjoV/xe1a757DDeZKlgd8Lp20fSDwvNt/Ejx8ueh3h3kuOtgDpIGSKX/l+XC+ltDpzjhh7bowI2/fOEf+kE53jvu9i4PiLnKdlY=
+