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/proc-macro-crate/.github/workflows/rust.yml

@@ -0,0 +1,55 @@
+name: Rust
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+env:
+  CARGO_TERM_COLOR: always
+
+jobs:
+  test:
+    name: cargo test
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: dtolnay/rust-toolchain@stable
+      - run: cargo test --all
+  msrv:
+    name: "Check MSRV: 1.82.0"
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v4
+    - name: Install Rust
+      uses: dtolnay/rust-toolchain@stable
+      with:
+        toolchain: 1.82.0  # MSRV
+    - uses: Swatinem/rust-cache@v2
+    - name: Default features
+      run: cargo test --all
+  rustfmt:
+    name: rustfmt
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v4
+    - name: Install Rust
+      uses: dtolnay/rust-toolchain@nightly
+      with:
+        toolchain: nightly
+        components: rustfmt
+    - uses: Swatinem/rust-cache@v2
+    - name: Check formatting
+      run: cargo fmt --all -- --check
+  clippy:
+    name: cargo clippy (forbidden methods)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: dtolnay/rust-toolchain@stable
+      # We just use clippy to spot forbidden methods.
+      # We disable the default lint set which has much questionable output.
+      - run: cargo clippy --all -- -A clippy::all -D clippy::disallowed_methods