feat(gfql): add public declarative schema model

[lmeyerov]

Closes #1337

Summary

• add stable public graphistry.schema declarations: NodeType, EdgeType, GraphSchema, EdgeTopology
• make node/edge property declarations Arrow-first: accept pyarrow.Schema, Arrow dtype/field mapping values, and GFQL RowSchema, while preserving int/str shorthand
• export node/edge declarations back to pyarrow.Schema with to_arrow() through the existing GFQL Arrow bridge
• support graphistry.bind(..., schema=schema) / g.bind(schema=schema) and adapt public schemas into the existing internal GraphSchemaCatalog
• extend Cypher preflight/binder validation for scoped node properties, scoped edge properties, declared relationship types, and edge source/destination topology
• wire schema-level strict/permissive policy: GraphSchema(strict=False) now makes schema-bound g.gfql_validate(...) permissive by default, while explicit call-level strict=True still forces strict validation
• document the public contract and add changelog/tests

Notes

• Does not complete GFQL type system follow-on B: schema inference API + typed topology extraction #1338 schema inference.
• Partially overlaps GFQL type system follow-on C: public schema-Arrow APIs + plottable boundary enforcement #1339 only for Arrow declaration/export helpers; GFQL type system follow-on C: public schema-Arrow APIs + plottable boundary enforcement #1339 should narrow to Arrow import, plottable-boundary enforcement, and coercion.

Validation

• python3 -m pytest -q graphistry/tests/compute/gfql/test_public_schema.py graphistry/tests/compute/gfql/cypher/test_binder.py::test_binder_schema_metadata_can_disable_schema_field_checks graphistry/tests/compute/gfql/cypher/test_binder.py::test_binder_schema_enforced_even_when_legacy_strict_name_flag_false -> 14 passed
• python3 -m pytest -q graphistry/tests/compute/gfql/test_public_schema.py graphistry/tests/compute/gfql/test_ir_arrow_bridge.py graphistry/tests/compute/gfql/cypher/test_binder.py -> 100 passed, 10 warnings
• ./bin/ruff.sh graphistry/compute/gfql_validate.py graphistry/compute/gfql/frontends/cypher/binder.py graphistry/tests/compute/gfql/test_public_schema.py graphistry/tests/compute/gfql/cypher/test_binder.py graphistry/schema.py -> pass
• ./bin/typecheck.sh graphistry/compute/gfql_validate.py graphistry/compute/gfql/frontends/cypher/binder.py graphistry/schema.py -> pass
• python3 -m compileall -q graphistry/compute/gfql_validate.py graphistry/compute/gfql/frontends/cypher/binder.py graphistry/schema.py -> pass
• git diff --check origin/master...HEAD -> pass
• Rebased onto origin/master@94577717; final PR head 3f3319d9
• Review skill reconverged after the strict/permissive policy fix: wave 10 found/fixed the IMPORTANT strict policy gap; waves 11/12 found no further issues; waves 13/14 reconverged after the final rebase
• DGX RAPIDS/cuDF on 3f3319d9: 25.02/gfql PASS, 26.02/gfql PASS; selected suite showed 366 passed, 5 warnings
• GitHub CI green on 3f3319d9: duplicate tck-gfql, docs, ReadTheDocs, full AI, core Python, pandas/polars, CodeQL, and no-plans-in-repo all passing

Local ./bin/test-minimal-lite.sh was attempted earlier through a temporary python -> python3 shim and failed in this CPU shell on local CUDA/cuDF initialization (cudaErrorNoDevice) plus GPU-backed UMAP/text paths. GPU validation has since passed on dgx-spark.