Add end-to-end test for ss-redir transparent proxy via QEMU

Boot a minimal Alpine Linux guest in QEMU with iptables, ss-redir, and
ss-nat to verify the full transparent proxy chain: iptables REDIRECT →
ss-redir → ss-server (host) → internet. The host-built ss-redir binary
and its shared library dependencies are copied into the Alpine rootfs,
avoiding static linking issues. Uses QEMU user-mode networking with KVM
acceleration when available. Linux-only CI step with 8-minute timeout.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/build.yml

@@ -45,3 +45,10 @@ jobs:
       - name: Stress test
         run: |
           python3 tests/stress_test.py --bin build/shared/bin/ --size 10
+
+      - name: ss-redir transparent proxy test (QEMU)
+        if: runner.os == 'Linux'
+        run: |
+          sudo apt-get install -y qemu-system-x86
+          bash tests/test_redir_qemu.sh build/shared/bin/
+        timeout-minutes: 8