Phase 2 kickoff: deletion contracts for #1250/#1252

[lmeyerov]

Summary

Phase 2 kickoff PR for the remaining pygraphistry work that unlocks graphistrygpt helper deletions:

• Plottable.from_dataset_id(id, token) helper to fetch existing dataset metadata + bindings #1252 from_dataset_id metadata hydration helper
• Declarative encode_*_from_dict constructor parity with React-side ['col','categorical', mapping] shape #1250 declarative React-shape encoding dispatcher parity

This kickoff commit establishes the deletion-contract-first scope and implementation slicing plan.

Why

Downstream deletion depends on explicit upstream contracts. This PR starts with those contracts as implementation guardrails so follow-up commits can be reviewed against concrete delete outcomes.

Included in this kickoff commit

• docs/roadmap/phase2-1250-1252-deletion-contract.md

Planned slices on this PR

1. Slice A (Plottable.from_dataset_id(id, token) helper to fetch existing dataset metadata + bindings #1252): implement from_dataset_id + hydration tests
2. Slice B (Declarative encode_*_from_dict constructor parity with React-side ['col','categorical', mapping] shape #1250): implement declarative encoding dispatcher + validation tests
3. Slice C: docs/changelog + explicit downstream deletion mapping

Related

• Closes Declarative encode_*_from_dict constructor parity with React-side ['col','categorical', mapping] shape #1250
• Closes Plottable.from_dataset_id(id, token) helper to fetch existing dataset metadata + bindings #1252
• Prior phase landed in Phase 1: settings validators + deep ring axis validation + persist axis defaults #1280