Address PR review feedback

- Move optimizer backends and viz libs from test feature to base deps
- Create separate typecheck env with mypy + type stubs
- Slim down docs feature to only sphinx tooling + doc-specific deps
- Slim down default env to just base + test (no typecheck/docs)
- Rename docs task to build-docs
- Remove pixi install from RTD config (pixi run handles it)
- Remove pre-commit/pdbp from pixi deps (install globally)
- Update contributor guide: explain global pre-commit install
- Revert unrelated algorithms.py changes from pre-commit hook
- Fix two pre-existing mypy errors in history.py and slice_plot_3d.py
- Pin pandas<3 for gradient-free-optimizers compatibility
- Pin gradient-free-optimizers>=1.6.0

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: timmens