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/bitflags-1.3.2/.github/workflows/rust.yml

@@ -0,0 +1,56 @@
+name: Rust
+
+on: [push, pull_request]
+
+env:
+  CARGO_TERM_COLOR: always
+
+jobs:
+  check:
+    name: Test
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: true
+      matrix:
+        rust:
+          - stable
+          - beta
+          - nightly
+          - 1.46.0
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v2
+
+      - name: Install Rust toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: ${{ matrix.rust }}
+          override: true
+
+      - name: Default features
+        uses: actions-rs/cargo@v1
+        with:
+          command: test
+          args: --features example_generated
+
+  embedded:
+    name: Build (embedded)
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v2
+
+      - name: Install Rust toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: nightly
+          target: thumbv6m-none-eabi
+          override: true
+
+      - name: Default features
+        uses: actions-rs/cargo@v1
+        with:
+          command: build
+          args: -Z avoid-dev-deps --features example_generated --target thumbv6m-none-eabi