Build script was Windows-only (cmd.exe) — made cross-platform in v4 onboarding; review #4

Filed per the standing "platform-specific → track it" rule.

What was found

The original build script used cmd.exe-only syntax and can't run on non-Windows:

rmdir /s /q dist
for %f in (dist\*.js) do ren "%f" "*.mjs"

The v4 flow runs npm run build (coverage-badge job) and npm run build:ci (publish) on Linux runners, so a Windows-only build breaks CI and publishing.

What was done (in PR #2, needs your review)

Made it cross-platform, minimally, with identical output layout:

• clean → a node -e "fs.rmSync('dist', {recursive:true,force:true})" one-liner
• one esbuild pass with --out-extension:.js=.mjs --outbase=src (dropped the now-unneeded rename-dist-js-to-mjs step)

Output matches the previous main/bin/exports (dist/*.mjs + dist/platform/*.mjs).

Ask

• Confirm the cross-platform build is acceptable (or adjust if you preferred a different toolchain).
• Sanity-check there are no other Windows-only assumptions in the build/dev scripts.

No product (src/) code changed. (Related: the source itself has intentional posix/win branches — those are correct cross-platform handling, not a portability bug.)