fix: wipe dist/ext at the start of build:ext

[manwithacat]

Description

build:ext copied src/ext/*.js into dist/ext but never wiped the directory. After a rename, the old basename stayed in the published tarball.

On 4.0.0 that shows up as:

• https://cdn.jsdelivr.net/npm/htmx.org@4.0.0/dist/ext/hx-optimistic.min.js → 404 (hx-optimistic was renamed to hx-pending in rename hx-optimistic to hx-pending #3958; only an orphan .map shipped)
• leftover dist/ext/hx-compat.* with no src/ext/hx-compat.js

This PR is just rm -rf dist/ext at the start of build:ext, per @1cg. No runtime change. dist/ is not in this PR; the next release build drops the orphans from the tarball.

Corresponding issue: #4041

Not asking to move npm latest off 2.x.

Testing

npm run build:ext now starts from an empty dist/ext, so only current src/ext/*.js names are emitted.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (four-dev for htmx 4 source)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly approved via an issue
• I ran the full browser test suite locally (npm run test) — not applicable; this is a one-line build-script change

Optional follow-ups (not in this PR)

Dropped the checker, CI job, and dev/RELEASE.md notes from the first version. Recording them here in case they are useful later:

1. Release-only completeness check — a tag / workflow_dispatch job that compares committed dist/ext to src/ext without rebuilding. A PR-time bun run build then check always passes once the wipe is in, so it would not gate the published tarball. A tag check would catch “forgot to rebuild dist/ before publish.” Keep any such script out of src/scripts/ (build:scripts copies *.py into the npm package).
2. Same wipe for other generated trees — build:editors, build:skills, and build:scripts are also copy-without-clean. A single rm -rf dist && mkdir -p dist at the start of npm run build would cover them, still with no extra tooling.
3. dev/RELEASE.md — npm build is a typo for npm run build. While 2.x is npm latest, 4.x needs npm publish --tag next. gh release create --prerelease when the version contains - keeps GitHub Latest on a real GA tag (v4.0.0-beta3 through beta5 were not marked prerelease).
4. Changelog on the next 4.x — hx-optimistic was renamed to hx-pending (rename hx-optimistic to hx-pending #3958); the old CDN path 404s.

[1cg]

Hey @manwithacat I like the cleaning out the dist on build but don't think we need the rest of the infrastructure. Can you update that?

[manwithacat]

Updated — this PR is now just rm -rf dist/ext at the start of build:ext. Dropped the checker, the CI job, and the RELEASE.md notes.

The next release build will drop the leftover hx-optimistic / hx-compat files from the tarball. I left a couple of optional follow-ups at the bottom of the description if they ever become useful.