ci: add Linux multi-distro matrix and coverage reporting (TODO-052,053)

Add Docker-based Linux build matrix (Ubuntu, Fedora, Arch) for
platform-specific issue detection. Add cargo-llvm-cov coverage
job with artifact upload.

.github/workflows/ci.yml

@@ -181,3 +181,67 @@ jobs:
       - uses: Swatinem/rust-cache@v2
       - name: cargo check --workspace
         run: cargo check --workspace --locked
+
+  linux-multi-distro:
+    name: Linux build (${{ matrix.distro }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - distro: ubuntu
+            image: ubuntu:24.04
+            install: |
+              apt-get update
+              apt-get install -y curl build-essential pkg-config \
+                libasound2-dev libpulse-dev libdbus-1-dev libsdl2-dev libopus-dev
+          - distro: fedora
+            image: fedora:latest
+            install: |
+              dnf install -y curl gcc pkg-config \
+                alsa-lib-devel pulseaudio-libs-devel dbus-devel SDL2-devel opus-devel
+          - distro: arch
+            image: archlinux:latest
+            install: |
+              pacman -Syu --noconfirm
+              pacman -S --noconfirm curl base-devel pkg-config \
+                alsa-lib pulseaudio dbus sdl2 opus
+    container:
+      image: ${{ matrix.image }}
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install system dependencies
+        run: ${{ matrix.install }}
+      - name: Install Rust
+        run: |
+          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
+          echo "$HOME/.cargo/bin" >> "$GITHUB_PATH"
+      - name: cargo check --workspace
+        run: cargo check --workspace
+
+  coverage:
+    name: cargo llvm-cov
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: System deps
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y \
+            libasound2-dev libpulse-dev pkg-config \
+            libdbus-1-dev libsdl2-dev libopus-dev
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          components: llvm-tools-preview
+      - uses: Swatinem/rust-cache@v2
+      - name: Install cargo-llvm-cov
+        run: cargo install --locked cargo-llvm-cov
+      - name: Generate coverage
+        env:
+          CHANORA_DISABLE_KEYRING: "1"
+        run: cargo llvm-cov --workspace --lcov --output-path lcov.info
+      - name: Upload coverage artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: lcov-report
+          path: lcov.info