Idea: replace black with ruff format — collapses two formatter passes into one

[egoran2]

TL;DR

Adopt ruff format and drop black. Same output (ruff format is intentionally byte-compatible with black), one tool, faster CI.

Why

The existing setup uses black + (via my earlier idea ticket) potentially ruff for lint. That means CI pulls two formatters, runs two passes, and contributors install both.

ruff format (stable since 0.1) is:

• Byte-compatible with black for the formatter rules already in use here
• ~30× faster on a clean cache (matters less for this repo's size, but the import-sort + format combined pass is noticeable)
• Backed by the same project that's already going to be invoked for lint, so configuration lives in one [tool.ruff] block in pyproject.toml

Migration sketch

1. pip install ruff (already a dev dep if the lint idea lands)
2. ruff format src/ tests/ and confirm zero diff vs black
3. Replace black invocations in Makefile and CI with ruff format and ruff format --check
4. Drop black from [project.optional-dependencies].dev
5. Update CONTRIBUTING.md

Open questions

• Does the existing pyproject.toml set any non-default black config (line length etc.) that we'd need to mirror in [tool.ruff.format]?
• Anyone strongly attached to black for non-CI workflows (editor integration etc.)?

Happy to put up the PR if maintainers are receptive.