feat(gfql): add schema Arrow boundary validation

[lmeyerov]

Closes #1339.
Refs #1058.

Summary

• Add experimental public schema↔Arrow import/export APIs:
  • NodeType.from_arrow(...)
  • EdgeType.from_arrow(...)
  • EdgeTopology.from_metadata(...)
  • GraphSchema.node_arrow(), edge_arrow(), to_arrow(), from_arrow(...)
• Add opt-in bound-schema Arrow boundary validation/coercion:
  • g.to_arrow(..., schema_validate='strict'|'autofix', schema_table='edges'|'nodes')
  • g.plot(..., schema_validate='strict'|'autofix')
  • g.upload(..., schema_validate='strict'|'autofix')
  • g.validate_arrow_schema('edges'|'nodes', validate='strict'|'autofix')
• Preserve default behavior: schema_validate=False by default, so existing plot()/upload()/to_arrow() calls are unchanged.
• Document the Arrow boundary recipe and update the changelog.

Design Notes

• This is declaration import/export, not inference. GraphSchema.from_arrow(...) requires named node/edge declarations and edge topology, either directly or from the payload emitted by GraphSchema.to_arrow().
• This does not implement gfql_remote() schema transport; GFQL remote: send bound typed GraphSchema with gfql_remote requests #1465 remains the remote follow-on.
• Boundary enforcement runs after the existing pandas/cuDF-to-Arrow conversion path, so it composes with validate='autofix'.
• schema_validate='strict' rejects missing declared columns, incompatible Arrow types, and non-null violations.
• schema_validate='autofix' casts compatible Arrow columns to declared Arrow types after normal conversion.
• Active node/edge type fragments are selected by true label__... markers so inactive relationship types do not force their properties.
• Multi-type table validation treats non-nullability as type-local, avoiding false failures on rows for other active labels/types.
• String/large-string and binary/large-binary are treated as strict-compatible logical families; autofix still casts to the declared physical Arrow type.
• Experimental marking from feat(gfql): add public declarative schema model #1457 is preserved.

Coordination

• GFQL type system follow-on B: schema inference API + typed topology extraction #1338 inference is not absorbed here. Inference output can produce feat(gfql): add public declarative schema model #1457 GraphSchema instances and then flow into this boundary layer.
• GFQL remote: send bound typed GraphSchema with gfql_remote requests #1465 remote transport is not absorbed here. The graph-level Arrow declaration payload is local/public API groundwork, not wire transport.
• Compiler-plan surface touched: no. No graphistry/compute/gfql/ir/** files changed.

LOC Buckets

• Production: +683 / -27, net +656 LOC
• Tests: +180 / -2, net +178 LOC
• Docs / changelog: +52 / -4, net +48 LOC

Validation

• Rebased over origin/master@79a61f9ab7d3efa4984007c0ae30660adbd6df4e; PR head 76065d0b4f20034ced71fd49485d94346323f662
• python3 -m pytest -q graphistry/tests/test_plotter.py::TestPlotterArrowConversions graphistry/tests/compute/gfql/test_public_schema.py -> 50 passed, 6 skipped, 1 xfailed
• python3 -m pytest -q docs/test_doc_examples.py -k schema -> 1 passed, 29 deselected
• python3 -m pytest -q graphistry/tests/compute/gfql/test_public_schema.py -> 21 passed
• ./bin/ruff.sh graphistry/schema.py graphistry/PlotterBase.py graphistry/Plottable.py graphistry/models/types.py graphistry/exceptions.py graphistry/tests/compute/gfql/test_public_schema.py -> pass
• ./bin/typecheck.sh graphistry/schema.py graphistry/PlotterBase.py graphistry/Plottable.py graphistry/models/types.py graphistry/exceptions.py -> pass
• CI typing follow-up: uvx --from mypy==1.14.1 mypy graphistry/PlotterBase.py -> pass after py3.8 mypy-compatible Arrow column variable/cast fix
• git diff --check origin/master...HEAD -> pass
• DGX RAPIDS 26.02: RAPIDS_VERSION=26.02 PROFILE=basic WITH_GPU=1 WITH_IMAGE_BUILD=0 TEST_FILES="graphistry/tests/test_plotter.py::TestPlotterArrowConversions graphistry/tests/compute/gfql/test_public_schema.py" docker/test-rapids-official-local.sh -> cudf 26.02.01, cuml 26.02.00, Python 3.13.12, 56 passed, 1 xfailed
• DGX RAPIDS 25.02: RAPIDS_VERSION=25.02 PROFILE=basic WITH_GPU=1 WITH_IMAGE_BUILD=0 TEST_FILES="graphistry/tests/test_plotter.py::TestPlotterArrowConversions graphistry/tests/compute/gfql/test_public_schema.py" docker/test-rapids-official-local.sh -> cudf 25.02.02, cuml 25.02.01, Python 3.12.9, 56 passed, 1 xfailed
• Review skill converged:
  • Wave 1 fixed type-local nullability for heterogeneous tables
  • Wave 2 fixed merged Arrow schema metadata preservation
  • Waves 3 and 4 clean

Pending / CI

• Full PR CI pending.