feat(simd): x86_64 baseline of SSE4.2, architecture-aware USE_SIMD

[lgritz]

USE_SIMD defaulted to empty, so we inherited whatever the compiler chose: SSE2 for gcc/clang on Linux, penryn (SSE4.1, no 4.2) for Apple clang, and plain SSE2 for MSVC, which never predefines the SSE* macros that simd.h keys off and which no Windows CI job overrides. Every x86-64 CPU has had SSE4.2 since 2008, so we were leaving the 32 OIIO_SIMD_SSE >= 4 fast paths on the floor for no reason.

Default USE_SIMD to sse4.2 when targeting x86_64, and leave it empty elsewhere -- NEON is architecturally mandatory on ARMv8-A, so aarch64 needs no help. USE_SIMD=0 still disables everything.

The block also had no notion of the target architecture at all: -m${feature} was a blind passthrough, so USE_SIMD=avx2,f16c on ARM emitted -mavx2 and failed to compile, which is why every ARM CI job leaves USE_SIMD unset. Now tokens are classified, tokens for the wrong CPU family are skipped with a status message, ARM tokens (neon, armv8.2-a+fp16, apple-m1, mcpu=...) are understood, and universal Apple builds qualify the x86 flags with -Xarch_x86_64 rather than handing -msse4.2 to the arm64 slice.

For MSVC on x64, emit the SSE4 defines unconditionally rather than only when the user names those tokens, and let simd.h reach the same conclusion on its own so that downstream users of the installed header benefit too. This is safe because MSVC exposes every intrinsic through AVX2 regardless of /arch:. gcc and clang do gate intrinsics on -m flags, so the header cannot do the same for them; that limitation is now documented in simd.h and INSTALL.md.

Assisted-by: Claude Code / claude-opus-5