[bfierz]

CI only ever built/tested the default SSE 4.2 backend. This adds coverage for the rest of the WEAVER_VECTORIZE matrix, plus a small test-fixture cleanup.

• Dedupe WEAVER_SIMD_{BOOLS,FLOATS,INTS} fixture macros out of bool.cpp/float.cpp/int.cpp into shared src/tests/common_{bool,float,int}.h headers.
• CI now builds+tests Generic/SSE 4.2/AVX/AVX2/AVX512 (Emscripten stays SSE-4.2-max only). AVX-512 skips gracefully if the runner's CPU doesn't support it.
• New build-arm job cross-compiles the NEON backend (armv7hf + armv8) via a Conan host profile and runs the tests under qemu-user, following the pattern already used for the Emscripten leg.