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/semver/.travis.yml

@@ -0,0 +1,18 @@
+language: rust
+rust:
+  - nightly
+  - beta
+  - stable
+sudo: false
+script:
+  - cargo build --verbose
+  - cargo test --verbose --features ci
+addons:
+  apt:
+    sources:
+      - kalakris-cmake
+    packages:
+      - cmake
+notifications:
+  email:
+    on_success: never