build: alias _Static_assert directory-wide for test targets (macOS + GCC)

The macOS 26 SDK's <mach/*.h> arm64 size-assert macros expand to
_Static_assert(...), which g++ rejects in C++ mode (only clang accepts it as
an extension). The agentty target already aliases _Static_assert=static_assert,
but PRIVATE, so the test/bench executables — which link the full IO/tool stack
(subprocess.cpp pulls <spawn.h> → mach headers) — never inherited it and failed
to compile. Apply the alias directory-wide inside the AGENTTY_BUILD_TESTS block
so every test target gets it. Unblocks the seam/freeze/render probe suite under
GCC; AppleClang doesn't need it (and can't build this tree — no cxx_std_26).

Author: 1ay1