fix(coverage): use atomic counters for worker threads

[Zxilly]

OpenWrt QEMU coverage intermittently fails with negative counts in proxy.c when worker threads update shared gcov counters. Compile coverage builds with -fprofile-update=atomic and keep lcov negative-count validation enabled.

Validation: GCC 14.2 passes all 123 existing tests and real REDIRECT/TPROXY netns integration; Clang 20 accepts the flag. In 100 real worker startup/shutdown cycles plus netns tests, both builds recorded 1,604 worker entries. The original build recorded only 3,207 listener calls (expected 3,208); the atomic build recorded all 3,208 and passed lcov. This local run demonstrated lost counts without reproducing the negative-count error itself.

Historical QEMU runs 34099489032 and 32688567132 show the same negative-count failure before the dependency update. Both OpenWrt 24.10.8 and 25.12.5 QEMU jobs now pass, including strict lcov generation and coverage upload.