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/proc-macro-error/.travis.yml

@@ -0,0 +1,19 @@
+language: rust
+rust:
+  - stable
+  - beta
+  - nightly
+script:
+  - cargo test --all
+matrix:
+  include:
+    - rust: 1.31.0
+      script: cargo test --tests # skip doctests
+  allow_failures:
+    - rust: nightly
+  fast_finish: true
+
+
+notifications:
+  email:
+    on_success: never