chore!: simplify install extras for 0.2.1

[maclandrol]

0.2.0 is already published to PyPI (immutable), so this ships as 0.2.1.

Extras simplification

• safe-mol (base) now includes molecule visualization alongside the notation core — still no PyTorch, so encode/decode/split work everywhere (incl. Mac Intel).
• safe-mol[model] is now the single ML extra: SAFE-GPT inference + the safe-train CLI + Weights & Biases logging.
• Removed the redundant/confusing [train], [viz], [wandb] and [all] extras. ([model] already installs everything, so [all] had no purpose.)
• Fixed the lazy-import hints in safe/__init__.py (model/tokenizer/wandb → safe-mol[model]; trainer → safe-mol[model]), and updated the workflows ([docs,train]/[test,docs,all] → […,model]), env.yml, README/docs install tables, migration guide, and CHANGELOG.

Verified

• Base-only install (no torch): import safe, encode/decode round-trip, split, and to_image all work; model APIs raise a clean pip install "safe-mol[model]" error.
• Core CI matrix command passes in a base+test env (132 tests, no torch); full suite passes with the model stack (165 tests).