feat: add OpenWrt cross-compilation and packaging (Phase F7)

- packaging/openwrt/: opkg Makefile, procd init script, uci config
- scripts/cross-compile.sh: build for musl targets with size checks
- .github/workflows/openwrt.yml: CI cross-compile + 5 MB size gate
- docs/openwrt.md: installation and configuration guide
- Targets: x86_64-musl, armv7-musleabihf, aarch64-musl
- Uses cargo-zigbuild for Docker-free cross-compilation

.github/workflows/openwrt.yml

@@ -0,0 +1,65 @@
+name: OpenWrt Cross-Compile
+
+on:
+  push:
+    tags:
+      - 'v*'
+  workflow_dispatch:
+
+env:
+  CARGO_TERM_COLOR: always
+  MAX_SIZE_MB: 5
+
+jobs:
+  cross-compile:
+    name: Cross-compile (${{ matrix.target }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        target:
+          - x86_64-unknown-linux-musl
+          - armv7-unknown-linux-musleabihf
+          - aarch64-unknown-linux-musl
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install Rust toolchain
+        uses: dtolnay/rust-toolchain@stable
+
+      - name: Install cargo-zigbuild and zig
+        run: |
+          pip3 install ziglang
+          cargo install cargo-zigbuild
+
+      - name: Add target
+        run: rustup target add ${{ matrix.target }}
+
+      - name: Build (size-optimised)
+        env:
+          CARGO_PROFILE_RELEASE_OPT_LEVEL: s
+          CARGO_PROFILE_RELEASE_LTO: 'true'
+          CARGO_PROFILE_RELEASE_CODEGEN_UNITS: '1'
+          CARGO_PROFILE_RELEASE_STRIP: symbols
+        run: |
+          cargo zigbuild --release --target ${{ matrix.target }} --bin qpq-server
+
+      - name: Check binary size
+        run: |
+          BINARY="target/${{ matrix.target }}/release/qpq-server"
+          SIZE=$(stat -c%s "$BINARY")
+          SIZE_MB=$(echo "scale=2; $SIZE / 1048576" | bc)
+          echo "Binary size: ${SIZE_MB} MB"
+          MAX_BYTES=$(( ${{ env.MAX_SIZE_MB }} * 1048576 ))
+          if [ "$SIZE" -gt "$MAX_BYTES" ]; then
+            echo "::error::Binary exceeds ${MAX_SIZE_MB} MB limit (${SIZE_MB} MB)"
+            exit 1
+          fi
+
+      - name: Upload artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: qpq-server-${{ matrix.target }}
+          path: target/${{ matrix.target }}/release/qpq-server
+          retention-days: 30