Drop scVelo as hard dependency, add as optional pip extra

[Marius1311]

Final step of the scVelo removal: drop scVelo from hard dependencies and make it an optional pip extra for velocity embedding stream/grid plots.

Changes

pyproject.toml

• Remove scvelo>=0.3 from [project] dependencies
• Add optional-dependencies.scvelo = ["scvelo>=0.3"] — used by TmatProjection.plot() for scv.pl.velocity_embedding_stream/grid (already import-guarded via _check_module_importable)
• Keep scvelo>=0.3 in [dependency-groups] test — needed for 5 projection tests

Source docstrings & comments

• Fix stale scvelo.pl.scatter → scanpy.pl.embedding ref in _base_kernel.py random walk kwargs docstring
• Drop scvelo compat: prefix from 4 legend-loc normalization comments (code uses scanpy directly now)
• Remove scvelo/issues/673 URL comment from _term_states_estimator.py (bug is in CellRank's own code)

Tests

• Move top-level import scvelo in tests/_helpers.py inside _create_dummy_adata() — only used for offline test data regeneration, never at test runtime

What stays

• Attribution comments ("copied from scvelo", "adapted from scvelo") — kept for credit
• docs/conf.py intersphinx mapping for scvelo — projection docstrings still cross-reference it
• tests/conftest.py try/except scvelo import — sets figdir for projection tests
• tests/test_plotting.py test_scvelo_* test names — historical, functional

Closes the scVelo removal plan (.github/prompts/PLAN_scvelo_removal.md).
Follows #1301 and #1302.