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

scripts/cross-compile.sh

@@ -0,0 +1,91 @@
+#!/usr/bin/env bash
+# Cross-compile quicproquo for musl targets (OpenWrt / embedded Linux).
+#
+# Produces statically linked, stripped binaries optimised for size.
+# Requires: cargo-zigbuild (preferred) or cross.
+#
+# Usage:
+#   ./scripts/cross-compile.sh                    # all targets
+#   ./scripts/cross-compile.sh x86_64-unknown-linux-musl  # single target
+#
+# Output: target/<triple>/release/qpq-server (stripped)
+
+set -euo pipefail
+
+TARGETS=(
+    x86_64-unknown-linux-musl
+    armv7-unknown-linux-musleabihf
+    aarch64-unknown-linux-musl
+)
+
+MAX_SIZE_MB=5
+
+# Size-optimised release profile overrides.
+export CARGO_PROFILE_RELEASE_OPT_LEVEL=s
+export CARGO_PROFILE_RELEASE_LTO=true
+export CARGO_PROFILE_RELEASE_CODEGEN_UNITS=1
+export CARGO_PROFILE_RELEASE_STRIP=symbols
+
+# Detect build tool.
+if command -v cargo-zigbuild &>/dev/null; then
+    BUILD_CMD="cargo zigbuild"
+elif command -v cross &>/dev/null; then
+    BUILD_CMD="cross build"
+else
+    echo "ERROR: Install cargo-zigbuild or cross first:" >&2
+    echo "  cargo install cargo-zigbuild   # recommended" >&2
+    echo "  cargo install cross            # alternative" >&2
+    exit 1
+fi
+
+# If arguments provided, use those as targets.
+if [ $# -gt 0 ]; then
+    TARGETS=("$@")
+fi
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+PROJECT_ROOT="$(cd "$SCRIPT_DIR/.." && pwd)"
+cd "$PROJECT_ROOT"
+
+echo "=== quicproquo cross-compilation ==="
+echo "Build tool: $BUILD_CMD"
+echo "Targets: ${TARGETS[*]}"
+echo ""
+
+FAILED=()
+for target in "${TARGETS[@]}"; do
+    echo "--- Building for $target ---"
+    if $BUILD_CMD --release --target "$target" --bin qpq-server; then
+        BINARY="target/$target/release/qpq-server"
+        if [ -f "$BINARY" ]; then
+            SIZE=$(stat -c%s "$BINARY" 2>/dev/null || stat -f%z "$BINARY")
+            SIZE_MB=$(echo "scale=2; $SIZE / 1048576" | bc)
+            MAX_BYTES=$((MAX_SIZE_MB * 1048576))
+            echo "  Binary: $BINARY"
+            echo "  Size: ${SIZE_MB} MB"
+            if [ "$SIZE" -gt "$MAX_BYTES" ]; then
+                echo "  WARNING: Binary exceeds ${MAX_SIZE_MB} MB size limit!"
+                FAILED+=("$target (size: ${SIZE_MB} MB)")
+            else
+                echo "  OK: within ${MAX_SIZE_MB} MB limit"
+            fi
+        else
+            echo "  ERROR: Binary not found at $BINARY"
+            FAILED+=("$target (binary not found)")
+        fi
+    else
+        echo "  ERROR: Build failed for $target"
+        FAILED+=("$target (build failed)")
+    fi
+    echo ""
+done
+
+if [ ${#FAILED[@]} -gt 0 ]; then
+    echo "=== FAILURES ==="
+    for f in "${FAILED[@]}"; do
+        echo "  - $f"
+    done
+    exit 1
+fi
+
+echo "=== All targets built successfully ==="