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/scopeguard/.github/workflows/ci.yaml

@@ -0,0 +1,49 @@
+name: CI
+
+on:
+  - push
+  - pull_request
+
+env:
+  CARGO_TERM_COLOR: always
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        rust:
+          - stable
+          - beta
+          - nightly
+          - 1.20.0 # MSRV
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install Rust
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: ${{ matrix.rust }}
+          override: true
+      - name: Build
+        run: cargo build
+      - name: Run tests
+        run: cargo test
+      - name: Run tests (without default features)
+        run: cargo test --no-default-features
+
+  no_std:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Test no_std support
+        run: |
+          rustup target add thumbv6m-none-eabi
+          cargo build --no-default-features --target thumbv6m-none-eabi
+
+  format:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Check formatting
+        run: cargo fmt -- --check