Add Unix domain socket support and collapse reactor backends #643

Job	Run time
Changelog Summary	41s
FreeBSD 14.3 (Clang): C++20	5m 46s
FreeBSD 15.0 (Clang): C++20	7m 47s
Generate Matrix	6s
GCC 13: C++20,C++23	4m 35s
GCC 15: C++20,C++23 (asan+ubsan)	16m 5s
GCC 14: C++20,C++23	5m 23s
GCC 15: C++20,C++23	13m 26s
GCC 13: C++20,C++23 (arm64)	3m 24s
GCC 12: C++20	8m 18s
GCC 15: C++20,C++23 (tsan)	11m 18s
GCC 15: C++20,C++23 (coverage, static)	10m 55s
GCC 14: C++20,C++23 (arm64)	4m 0s
Clang 19: C++20,C++23	5m 53s
Clang 17: C++20	5m 0s
Clang 17: C++20 (arm64)	3m 47s
Clang 20: C++20,C++23 (asan+ubsan, static)	7m 29s
Clang 20: C++20,C++23 (tsan, static)	6m 18s
MSVC 14.34: C++20	2h 0m 8s
Clang 19: C++20,C++23 (arm64)	4m 1s
Clang 20: C++20,C++23 (arm64)	3m 33s
Clang 20: C++20,C++23 (x86, static)	8m 12s
Clang 18: C++20,C++23	5m 33s
Clang 20: C++20,C++23 (time-trace)	5m 0s
MSVC 14.42: C++20	7m 33s
Clang 20: C++20,C++23 (clang-tidy)	5m 19s
Clang-CL: C++20 (asan)	2h 0m 7s
MSVC 14.42: C++20 (asan)	2h 0m 10s
Apple-Clang: C++20,C++23 (macOS 26)	2h 0m 5s
Apple-Clang: C++20,C++23 (macOS 26, coverage, static)	53m 32s
Apple-Clang: C++20,C++23 (macOS 15)	2h 0m 7s
Apple-Clang: C++20,C++23 (macOS 26, tsan, static)	2h 0m 4s
Clang-CL: C++20	2h 0m 8s
MinGW Clang: C++20 (static)	2h 0m 8s
MinGW Clang: C++20 (coverage, static)	32m 37s
Apple-Clang: C++20,C++23 (macOS 26, asan+ubsan)	2h 0m 5s
GCC 15: C++20,C++23 (superproject CMake)	10m 44s
Clang 18: C++20,C++23 (arm64)	3m 33s
	22h 20m 50s

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