Portable SIMD intrinsics accepted behind a GOEXPERIMENT

The portable SIMD intrinsics proposal is now accepted behind a GOEXPERIMENT, giving Go a first cut at an architecture and vector-size agnostic API for hand-tuned numeric code, with sync/atomic also picking up a Uint64Pair for double-word CAS. On the tooling side, GoLand 2026.2 lands with in-IDE profiling, escape analysis and live CPU and memory charts, which pairs oddly well with a week's worth of accepted work aimed squarely at people who care what the hardware is actually doing.