fix: wipe dist/ext on build and check completeness in CI

[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 does not change runtime behaviour. It makes dist/ext a pure function of src/ext and fails CI if a future rename leaves leftovers.

Corresponding issue: #4041

Changes:

1. rm -rf dist/ext at the start of build:ext
2. src/scripts/check_dist.py — every src/ext/*.js has .js + .min.js; no dist basename that is not in src; every .map has a matching .js
3. CI dist job: check:dist:test, then bun run build, then check:dist
4. dev/RELEASE.md: run check:dist after build; npm publish --tag next while 2.x is npm latest; --prerelease on GitHub when the version contains -

dist/ is not in this PR. The next release build with the new build:ext will drop the orphans from the tarball. A changelog line on that release (hx-optimistic → hx-pending, old CDN path 404s) would help consumers.

Not asking to move npm latest off 2.x.

Testing

python3 src/scripts/check_dist.py --self-test   # pass
python3 src/scripts/check_dist.py               # fail on current four-dev leftovers (hx-optimistic, hx-compat)

The new CI dist job is the full-build gate (bun run build then check:dist). Runtime tests are unchanged.

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 change is the dist build/check path, covered by check:dist:test and the new CI job