chore: consolidate dev dependencies into PEP 735 groups

[ramakrishnap-nv]

Problem

Developer dependencies were declared in two places at once, under the same name:

[project.optional-dependencies]
dev = ["ruff==0.15.9", "pytest>=9.0", "pre-commit==4.3.0"]

[dependency-groups]
dev = ["ipykernel>=7.1.0"]

Two mechanisms, same name, different contents. Extras are also published in package metadata, so developer tooling was advertised as an optional runtime feature of the library.

Change

Install as	Contains
--extra	genuine optional runtime features — cuda12, cuda13, cuda13-socp
--group dev	ruff, pytest, pre-commit
--group notebooks	ipykernel

• Move ruff/pytest/pre-commit into the dev group; drop the dev extra.
• Set default-groups = []. uv installs the dev group by default, which would make an end-user or launchable uv sync --extra cuda13 pull ruff/pytest/pre-commit.
• Update README, CONTRIBUTING and the workflows; document the split in CONTRIBUTING.

A regression this surfaced

Disabling default-groups broke the documented install flow, and I caught it before pushing:

uv sync --extra cuda13
uv run python -m ipykernel install ...   # No module named ipykernel

ipykernel had been in the dev group and only worked because uv installs that group by default — the README's kernel step depended on that implicitly. Registering a Jupyter kernel is an end-user task, not a dev one, so ipykernel moved to its own notebooks group, and the two CI syncs followed by uv run python -m ipykernel install (main.yml:80, release-2512.yml:59) now request it. The GPU-pytest and CPU-test syncs correctly stay --group dev only.

Verification

Full group matrix, checked against a real venv:

uv sync                                ipykernel=n ruff=n pytest=n
uv sync --group notebooks              ipykernel=Y ruff=n pytest=n
uv sync --group dev                    ipykernel=n ruff=Y pytest=Y
uv sync --group notebooks --group dev  ipykernel=Y ruff=Y pytest=Y

• README flow verified end to end: sync, then register the kernel, succeeds.
• Tests pass on 3.11 and 3.12 through the --group dev path CI uses (60 passed, 3 skipped).
• uv lock --locked clean; pre-commit run --all-files clean.

Behaviour change worth noting in review: uv sync --extra dev now fails with

error: Extra `dev` is not defined in the project's `optional-dependencies` table

That is deliberate — a loud failure beats silently producing an environment without the tooling. The launchable notebooks are unaffected; they only ever used uv sync --extra {cuda_suffix}.

Merge ordering

Rebased onto main after #67, #69 and #72 merged. The release-2512.yml edit was dropped, since #67 deleted that file. Still conflicts with #68, which adds a notebooks group (papermill, nbconvert, ipykernel) and ci/utils scripts using --extra dev — the two are compatible in intent (#68 group is a superset), but whichever merges second needs a rebase.

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