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/once_cell/.github/workflows/ci.yaml

@@ -0,0 +1,28 @@
+name: CI
+on:
+  pull_request:
+  push:
+    branches: ["master", "staging", "trying"]
+
+env:
+  CARGO_INCREMENTAL: 0
+  CARGO_NET_RETRY: 10
+  CI: 1
+  RUST_BACKTRACE: short
+  RUSTFLAGS: -D warnings
+  RUSTUP_MAX_RETRIES: 10
+
+jobs:
+  test:
+    name: Rust
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        fetch-depth: 0 # fetch tags for publish
+    - uses: Swatinem/rust-cache@359a70e43a0bb8a13953b04a90f76428b4959bb6
+    - run: cargo run -p xtask -- ci
+      env:
+        CARGO_REGISTRY_TOKEN: ${{ secrets.CRATES_IO_TOKEN }}
+        MIRIFLAGS: -Zmiri-strict-provenance