`build:ext` does not clean `dist/ext`, so renamed extensions 404 (hx-optimistic at 4.0.0)

`build:ext` copies `src/ext/*.js` into `dist/ext` but never deletes the directory first. After a rename, the old basename stays in the published tarball (or an orphan `.map` does).

## Repro (4.0.0)

```
curl -I https://cdn.jsdelivr.net/npm/htmx.org@4.0.0/dist/ext/hx-optimistic.min.js
# 404

curl -I https://cdn.jsdelivr.net/npm/htmx.org@4.0.0/dist/ext/hx-pending.min.js
# 200
```

`hx-optimistic.js` was renamed to `hx-pending.js` in #3958. The 4.0.0 tree / npm pack still contains `dist/ext/hx-optimistic.min.js.map` with **no** matching `.js`. The same tree also still has a full leftover `hx-compat.*` set (`src/ext` only has `htmx-2-compat.js`).

Cause is mechanical: `package.json` `build:ext` is `mkdir -p dist/ext && for file in src/ext/*.js …` with no wipe. CI (`htmx 4 CI`) runs tests and `src/scripts/content/check.py`, but does not build dist or compare `src/ext` to `dist/ext`. `dev/RELEASE.md` also has no completeness check and no `--prerelease` for `-beta` / `-rc` tags (`v4.0.0-beta3` through `beta5` were published with `prerelease=false`; `beta6` was marked correctly).

## Ask

1. Wipe `dist/ext` at the start of `build:ext` so a release tarball is a pure function of `src/ext`.
2. Add a cheap completeness check (every `src/ext/*.js` has `.js` + `.min.js`; no dist basename that is not in src; every `.map` has a matching `.js`) and run it from CI after `bun run build`, and from the release checklist before `npm publish` / `gh release create`.
3. In `dev/RELEASE.md`: `npm publish --tag next` while 2.x remains npm `latest`; pass `--prerelease` on GitHub when the version contains `-`.

No runtime change. A changelog line on the next 4.x release that `hx-optimistic` is now `hx-pending` (old CDN path 404s) would help consumers. I am happy to send a `four-dev` PR for (1)–(3).

Not asking to move npm `latest` off 2.x — that policy is already documented.

Start with package.json and the build:ext command, then inspect the CI path that runs bun run build and src/scripts/content/check.py. Review dev/RELEASE.md for the publishing and prerelease checklist. Done means stale dist/ext files are excluded, completeness is checked in CI and release preparation, and the documented npm and GitHub release commands match the stated policy.

Written by the indexing model from the issue text.