feat(schemas): export tooling contract artifacts - #1590

Summary

• Add committed structural JSON Schema artifacts for encodings, React settings, and URL params.
• Add a stdlib graphistry.devschemas.export generator/checker and schema drift test coverage.
• Add a CI schema-artifacts drift guard plus a short API docs note.

Closes #1326

Artifacts

• schemas/encodings.schema.json
• schemas/react-settings.schema.json
• schemas/url-params.schema.json

Scope notes

• No hard pydantic/runtime dependency was added.
• JSON Schema artifacts are structural contracts. Existing PyGraphistry validators remain canonical for semantic validation.
• MANIFEST.in includes schemas/*.schema.json so source distributions carry the committed artifacts.

LOC buckets

• Production/tooling: +237 / -0, net +237 LOC
• Tests: +71 / -0, net +71 LOC
• Schema artifacts: +655 / -0, net +655 LOC
• CI/packaging: +20 / -0, net +20 LOC
• Docs/changelog: +19 / -0, net +19 LOC

Compiler-plan surface touched

No. This is schema artifact generation and public tooling metadata only; no GFQL compiler-plan semantics, route names, execution dispatch, diagnostics, schema/type-system runtime hooks, IR metadata, remote schema hooks, or compatibility executors were changed.

Validation

• python3 -m graphistry.devschemas.export --check -> passed
• python3 -m pytest -q graphistry/tests/test_schema_artifacts.py graphistry/tests/test_viz_settings.py graphistry/tests/test_validate_settings.py -> 28 passed
• python3 -m coverage run --data-file=/tmp/schema-artifacts.coverage -m pytest -q graphistry/tests/test_schema_artifacts.py && python3 bin/changed_line_coverage.py --data-file /tmp/schema-artifacts.coverage --base-ref origin/master --head-ref HEAD --min-percent 80 --output-dir /tmp/schema-artifacts-changed-coverage -> passed, 57/57 changed executable lines covered
• ./bin/ruff.sh graphistry/devschemas graphistry/tests/test_schema_artifacts.py -> passed
• ./bin/typecheck.sh graphistry/devschemas graphistry/tests/test_schema_artifacts.py -> passed
• git diff --check -> passed
• Review skill converged: wave 1 fixed sdist MANIFEST coverage; waves 2 and 3 no findings
• DGX/cuDF: skipped; no runtime, DataFrame, cuDF, or GPU path touched