all: REVERSE MERGE dev.simd (9ac524a) into master

This commit is a REVERSE MERGE.
It merges dev.simd back into its parent branch, master.

Merge List:

+ 2025-12-03 9ac524a [dev.simd] all: merge master (5945fc0) into dev.simd
+ 2025-12-03 8b41e84 [dev.simd] internal/buildcfg: default-disable simd experiment, for merge w/ master
+ 2025-12-03 608dba4 [dev.simd] simd: make "best" instruction choice also depend on commutativity
+ 2025-11-25 f3a3065 [dev.simd] simd, cmd/compile: add float -> float conversions
+ 2025-11-25 d6564ed [dev.simd] simd, cmd/compile: add int -> fp conversions
+ 2025-11-25 86cd9b5 [dev.simd] simd, cmd/compile: add float -> int conversions
+ 2025-11-24 1265ebf [dev.simd] internal/buildcfg: turn GOEXPERIMENT=simd back on

Change-Id: I05a1eedc406ba6e7668ff2101917a0adef656452

Author: dr2chase