feat: isolate visualization dependencies behind optional extra

[SkyeAv]

Moves visualization-only dependencies out of TCT's minimal installation and documents the opt-in installation path. The base package can now be installed without pulling plotting and graph-rendering packages.

Optional Visualization Dependencies

• vision extra: Moves ipycytoscape, matplotlib, networkx, pyvis, and seaborn into [project.optional-dependencies].vision.
• Lazy imports: Loads visualization libraries only when visualization functions are called, preserving base-package imports without those dependencies.
• Lockfile: Regenerates uv.lock with the new optional dependency markers.

Documentation

• Minimal install: Documents pip install TCT.
• Visualization install: Documents pip install "TCT[vision]" and uv sync --extra vision in README.md and docs/source/intro.md.

Testing

• uv lock --check — passed.
• uv run --extra mcp pytest -q — 58 passed, 1 skipped.
• uv build --out-dir /tmp/tct-dist-check — wheel and source distribution built successfully.
• Base import guarded against visualization imports — passed.
• Sphinx CI run — passed: run #31650717890.
• uv run ruff check TCT tests — existing repository findings remain; CI lint is non-blocking.