G3: migration guide 2.0->2.1 + release notes (write last)

[jepegit]

Epic G — Docs · Migration guide 2.0 → 2.1 + release notes. Write last (describes what shipped).

Acceptance

• Every Epic E removal with old→new mapping.
• The collectors cross-cell cycle-narrowing bug-fix before/after example.
• The batch symbol map (old batch_tools/entry points → new API; iterate_batches/process_batch recipes).
• Note: bokeh+seaborn backends dropped; prms.* removed, c.mass facade kept.

Depends on all of A–E landing. Part of #696.