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/display-interface/.github/workflows/ci.yml

@@ -0,0 +1,31 @@
+on:
+  push:
+    branches: [ staging, trying, master ]
+  pull_request:
+
+name: Continuous integration
+jobs:
+  ci_check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
+      - uses: actions-rs/cargo@v1
+        with:
+          command: build
+      - uses: actions-rs/cargo@v1
+        with:
+          command: build
+          args: --package display-interface-spi
+      - uses: actions-rs/cargo@v1
+        with:
+          command: build
+          args: --package display-interface-i2c
+      - uses: actions-rs/cargo@v1
+        with:
+          command: build
+          args: --package display-interface-parallel-gpio

vendor/display-interface/.github/workflows/clippy.yml

@@ -0,0 +1,32 @@
+on:
+  push:
+    branches: [ staging, trying, master ]
+  pull_request:
+
+name: Clippy check
+jobs:
+  clippy_check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
+          components: clippy
+      - uses: actions-rs/clippy-check@v1
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+      - uses: actions-rs/clippy-check@v1
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          args: --package display-interface-spi
+      - uses: actions-rs/clippy-check@v1
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          args: --package display-interface-i2c
+      - uses: actions-rs/clippy-check@v1
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          args: --package display-interface-parallel-gpio

vendor/display-interface/.github/workflows/rustfmt.yml

@@ -0,0 +1,23 @@
+on:
+  push:
+    branches: [ staging, trying, master ]
+  pull_request:
+
+name: Code formatting check
+
+jobs:
+  fmt:
+    name: Rustfmt
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
+          components: rustfmt
+      - uses: actions-rs/cargo@v1
+        with:
+          command: fmt
+          args: --all -- --check