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/.github/workflows/ci.yml

@@ -0,0 +1,42 @@
+on:
+  push:
+    branches: [main, staging, trying]
+  pull_request:
+    branches: [main]
+
+name: Continuous integration
+
+jobs:
+  test :
+    name: test
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        target:
+          - x86_64-unknown-linux-gnu
+          - i686-unknown-linux-musl
+
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          target: ${{ matrix.target }}
+          override: true
+
+      - uses: actions-rs/cargo@v1
+        with:
+          command: test
+          args: --target ${{ matrix.target }}
+
+  ci-success:
+    name: ci
+    if: github.event_name == 'push' && success()
+    needs:
+      - test
+    runs-on: ubuntu-latest
+    steps:
+      - name: Mark the job as a success
+        run: exit 0