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/cfg-if/.github/workflows/main.yaml

@@ -0,0 +1,48 @@
+name: CI
+on: [push, pull_request]
+
+permissions:
+  contents: read
+
+env:
+  RUSTDOCFLAGS: -Dwarnings
+  RUSTFLAGS: -Dwarnings
+
+jobs:
+  test:
+    name: Test
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        rust:
+          - "1.32" # msrv
+          - stable
+          - beta
+          - nightly
+    steps:
+      - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
+      - name: Install Rust ${{ matrix.rust }}
+        run: |
+          rustup self update
+          rustup update ${{ matrix.rust }}
+          rustup default ${{ matrix.rust }}
+          rustc -vV
+      - name: Run tests
+        run: |
+          set -eux
+          # Remove `-Dwarnings` at the MSRV since lints may be different
+          [ "${{ matrix.rust }}" = "1.32" ] && export RUSTFLAGS="--cfg msrv_test"
+          cargo test
+
+  rustfmt:
+    name: Rustfmt
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
+      - name: Install Rust Stable
+        run: |
+          rustup update stable
+          rustup default stable
+          rustup component add rustfmt
+      - name: Run rustfmt
+        run: cargo fmt -- --check

vendor/cfg-if/.github/workflows/publish.yaml

@@ -0,0 +1,25 @@
+name: Release-plz
+
+permissions:
+  pull-requests: write
+  contents: write
+
+on:
+  push: { branches: [main] }
+
+jobs:
+  release-plz:
+    name: Release-plz
+    runs-on: ubuntu-24.04
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v5
+        with:
+          fetch-depth: 0
+      - name: Install Rust (rustup)
+        run: rustup update nightly --no-self-update && rustup default nightly
+      - name: Run release-plz
+        uses: MarcoIeni/release-plz-action@v0.5
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          CARGO_REGISTRY_TOKEN: ${{ secrets.CARGO_REGISTRY_TOKEN }}