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/cortex-m/.github/CODEOWNERS

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+* @rust-embedded/cortex-m

vendor/cortex-m/.github/bors.toml

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+block_labels = ["needs-decision"]
+delete_merged_branches = true
+required_approvals = 1
+status = [
+  "ci-linux (stable)",
+  "ci-linux (1.38.0)",
+  "rustfmt",
+  "clippy",
+]

vendor/cortex-m/.github/workflows/ci.yml

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+on:
+  push:
+    branches: [ staging, trying, master ]
+  pull_request:
+
+name: CI
+
+jobs:
+  ci-linux:
+    runs-on: ubuntu-latest
+    continue-on-error: ${{ matrix.experimental || false }}
+    strategy:
+      matrix:
+        # All generated code should be running on stable now
+        rust: [stable]
+        features: ["", "critical-section-single-core"]
+
+        include:
+          # Test MSRV
+          - rust: 1.38.0
+            features: ""
+
+          # Test nightly but don't fail
+          - rust: nightly
+            features: ""
+            experimental: true
+          - rust: nightly
+            features: "critical-section-single-core"
+            experimental: true
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: ${{ matrix.rust }}
+          override: true
+      - name: Run tests
+        run: cargo test --all --features "${{ matrix.features }}"
+
+# FIXME: test on macOS and Windows

vendor/cortex-m/.github/workflows/clippy.yml

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+on:
+  push:
+    branches: [ staging, trying, master ]
+  pull_request_target:
+
+name: Clippy check
+jobs:
+  clippy:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        if: github.event_name == 'pull_request_target'
+        with:
+          ref: refs/pull/${{ github.event.number }}/head
+      - uses: actions/checkout@v2
+        if: github.event_name != 'pull_request_target'
+      - 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 }}
+          args: --features critical-section-single-core

vendor/cortex-m/.github/workflows/cron.yml

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+on:
+  schedule:
+    # Run every week at 8am UTC Saturday.
+    - cron: '0 8 * * SAT'
+
+name: Cron CI
+
+jobs:
+  ci-linux:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
+      - name: Run tests
+        run: cargo test --all
+      - uses: imjohnbo/issue-bot@v2
+        if: failure()
+        with:
+          title: CI Failure
+          labels: ci
+          body: |
+            Scheduled CI run failed. Details:
+
+            https://github.com/${{ github.repository }}/actions/runs/${{ github.run_id }}
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

vendor/cortex-m/.github/workflows/rustfmt.yml

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+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