packaging: republish python-pywal to [breadway]

python-pywal was dropped from Arch's [extra] repo (AUR-only now), so
`pacstrap` can no longer resolve it and every ISO build fails with
"target not found: python-pywal". The `wal` binary is load-bearing —
bread-theme shells out to it to extract the colour palette from the
user's wallpaper.

Republish it the same way as calamares / bibata / powerlevel10k /
yay-bin:

- `packaging/python-pywal/PKGBUILD` — in-house copy of the AUR PKGBUILD
  (Morten Linderud's), modernised to `python -m build` / `installer`
  instead of the removed `setup.py install`, sha256-only sources like
  the sibling PKGBUILDs. Test-built locally: 28 unit tests pass, package
  ships `/usr/bin/wal`.
- `.forgejo/workflows/python-pywal.yml` — builds + PUTs to the Forgejo
  Arch registry on a push to `packaging/python-pywal/**`.
- `signed-repo.yml` gains it as a `workflow_run` trigger;
  `ci-publish-signed-repo.sh` gains it in `PACKAGES` so the signed
  dl.breadway.dev/arch db picks it up.
- packages.x86_64 keeps the `python-pywal` line (now sourced from
  [breadway]) with a note.

Ordering: `python-pywal.yml` must publish to the registry once before
`signed-repo.yml` runs, or the collect step errors "registry db missing
packages: python-pywal".

packaging/arch/README.md

@@ -3,7 +3,7 @@ Arch packaging
 
 This directory only holds `PKGBUILD`s for third-party AUR packages BOS
 republishes to the `[breadway]` pacman repo (`calamares`, `bibata`,
-`powerlevel10k`, `yay-bin`) — not the user's own code. See each
+`powerlevel10k`, `yay-bin`, `python-pywal`) — not the user's own code. See each
 subdirectory's `.forgejo/workflows/<name>.yml` (in this repo) for how each
 one publishes on a push to `packaging/<name>/**`.