ci: add manually triggerable beta release workflow (#886)

## Summary

- Extract the beta release pipeline (`release_prepare` →
`changelog_update` → `pypi_publish` → `doc_release_post_publish`) from
`on_master.yaml` into a new `manual_release_beta.yaml` reusable
workflow.
- The new workflow exposes both `workflow_dispatch` (manual trigger from
the GitHub UI) and `workflow_call` (so `on_master` can keep
auto-publishing betas after master pushes).
- `on_master.yaml` now has a single `beta_release` job that calls the
new workflow, gated on the same `feat|fix|perf|refactor|style:` commit
prefixes and on `code_checks` + `tests` passing.

## Why

We previously had no way to cut a beta without pushing a
release-triggering commit to master. With this change, a beta can be
released on demand from the Actions tab, while the existing automatic
flow on master is preserved.