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/hash32/.travis.yml

@@ -0,0 +1,33 @@
+language: rust
+
+matrix:
+  include:
+    - env: TARGET=x86_64-unknown-linux-gnu
+    - env: TARGET=x86_64-unknown-linux-gnu
+      rust: nightly
+    - env: TARGET=thumbv7m-none-eabi
+      rust: nightly
+
+before_install: set -e
+
+install:
+  - bash ci/install.sh
+
+script:
+  - bash ci/script.sh
+
+after_script: set +e
+
+cache: cargo
+
+before_cache:
+  - chmod -R a+r $HOME/.cargo;
+
+branches:
+  only:
+    - staging
+    - trying
+
+notifications:
+  email:
+    on_success: never