Closes #135. Cleanly separates **extras** (PEP 621) from **dependency groups** (PEP 735), which were previously conflated through `PYTHON_EXTRA_DEPS`.

### Added

- **`dep-groups` / `PYTHON_DEP_GROUPS`** — dedicated input for PEP 735 groups. Wired for Poetry (`--only`), uv (`--group`), pip+pyproject (`--group`, requires pip ≥ 25.1), and PDM (`--group`).
- **`install-deps-opts` / `PYTHON_INSTALL_DEPS_OPTS`** — generic install-options input, appended verbatim to the install command for Poetry, uv, hatch, PDM, and pipenv (counterpart to the pip-only `pip-install-deps-opts`).
- `PIP_INSTALL_DEPS_OPTS` is now also appended for the **setuptools** and **reqfile** build-systems (previously: pip+pyproject only).

### Changed

- **Poetry**: install command gains `--only $PYTHON_DEP_GROUPS` when set; `--extras` behavior on `PYTHON_EXTRA_DEPS` unchanged.
- **pip+pyproject**: the `--group` loop now consumes `PYTHON_DEP_GROUPS` instead of `PYTHON_EXTRA_DEPS`.
- Documentation: harmonized `dep-groups` description across `kicker.json`, `gitlab-ci-python.yml`, and `README.md` (same build-system order, same Poetry link).

### Fixed

- **PDM + `PYTHON_EXTRA_DEPS`**: stop passing `--extra` to `pdm sync`, which is not a valid `pdm sync` option (PDM uses `-G`/`--group` for both PEP 621 optional-dependencies and PEP 735 groups). Setting `PYTHON_EXTRA_DEPS` with the PDM build-system used to fail; the path is now removed and `PYTHON_DEP_GROUPS` is wired to `--group` instead.
- **uv + `PYTHON_EXTRA_DEPS`**: multi-value handling. Was passed as a single `--extra "a b"` (a literal multi-word string uv would never resolve), now expanded as one flag per token: `--extra a --extra b`. `PYTHON_DEP_GROUPS` is wired in the same loop style with `--group`.
- **uv documentation**: the `extra-deps` link for uv pointed to *dependency-groups* by mistake; now points to *handling-of-extraneous-packages*.

### ⚠️ Micro breaking changes

- **pip+pyproject + `PYTHON_EXTRA_DEPS`**: no longer drives `--group`. The previous behavior used `PYTHON_EXTRA_DEPS` (despite the name) to feed PEP 735 `--group` flags to pip ≥ 25.1; this was documented but misnamed. Migrate values to `dep-groups` / `PYTHON_DEP_GROUPS`.

fix: split extras dependency and implemented the new "dependency-groups" standard.

Closes #135

See merge request !196

# [9.5.0](9.4.1...9.5.0) (2026-05-08)

### Features

* **deps:** split extras and adopt PEP 735 dependency-groups ([78798b1d](78798b1d))