Add Unix domain socket support and collapse reactor backends #642

Job	Run time
Changelog Summary	1m 15s
FreeBSD 15.0 (Clang): C++20	7m 53s
Generate Matrix	5s
FreeBSD 14.3 (Clang): C++20	5m 42s
GCC 13: C++20,C++23	5m 10s
GCC 15: C++20,C++23	14m 33s
GCC 15: C++20,C++23 (tsan)	14m 57s
Clang 17: C++20 (arm64)	3m 47s
GCC 13: C++20,C++23 (arm64)	4m 17s
Clang 17: C++20	5m 53s
GCC 12: C++20	9m 38s
GCC 15: C++20,C++23 (coverage, static)	9m 46s
Clang 18: C++20,C++23	5m 40s
GCC 14: C++20,C++23 (arm64)	4m 8s
Clang 19: C++20,C++23	5m 32s
GCC 15: C++20,C++23 (superproject CMake)	13m 12s
Clang 18: C++20,C++23 (arm64)	3m 43s
GCC 14: C++20,C++23	4m 59s
Clang 20: C++20,C++23 (clang-tidy)	5m 22s
Clang 19: C++20,C++23 (arm64)	4m 39s
GCC 15: C++20,C++23 (asan+ubsan)	18m 10s
MSVC 14.34: C++20	1h 11m 18s
Clang 20: C++20,C++23 (tsan, static)	6m 53s
Clang 20: C++20,C++23 (time-trace)	5m 9s
Apple-Clang: C++20,C++23 (macOS 26, asan+ubsan)	1h 11m 14s
MinGW Clang: C++20 (coverage, static)	31m 47s
Clang 20: C++20,C++23 (asan+ubsan, static)	7m 45s
Clang 20: C++20,C++23 (x86, static)	7m 20s
MSVC 14.42: C++20 (asan)	1h 11m 22s
Apple-Clang: C++20,C++23 (macOS 26)	1h 11m 14s
Clang-CL: C++20	1h 11m 16s
Apple-Clang: C++20,C++23 (macOS 26, tsan, static)	1h 11m 14s
Apple-Clang: C++20,C++23 (macOS 15)	1h 11m 19s
Apple-Clang: C++20,C++23 (macOS 26, coverage, static)	52m 21s
MinGW Clang: C++20 (static)	1h 11m 17s
Clang-CL: C++20 (asan)	1h 11m 18s
MSVC 14.42: C++20	1h 11m 21s
Clang 20: C++20,C++23 (arm64)	3m 39s
	16h 16m 8s

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