Release matrix: Linux, Windows, macOS and FreeBSD

Drop android (same binary as linux/arm64), big-endian ppc64 (cannot be
verified: even hello-world crashes under qemu-ppc64 with the cross
toolchain), NetBSD (cross 0.2.5's sysroot lacks libexecinfo), illumos,
Solaris and WASI, as agreed. docs/PLATFORMS.md records each decision.

Co-Authored-By: Claude <noreply@anthropic.com>

docs/MIGRATION.md

@@ -92,8 +92,8 @@ Command line:
 - `ls`/`list` piped into a reader that stops early end quietly with
   status 141, as Go's did by `SIGPIPE`.
 
-Platforms: see `PLATFORMS.md`. Some targets Go built have no stable Rust
-equivalent; they are listed there and need a decision before release.
+Platforms: releases now cover Linux, Windows, macOS and FreeBSD (16
+binaries, down from 43); `PLATFORMS.md` lists what was dropped and why.
 
 ## Unchanged on purpose