ci: contain schema-only test fanout

[lmeyerov]

Summary

• Adds a conservative schema_only path classifier for public schema runtime, schema artifact, and public schema test paths.
• Uses schema_only through narrow_python_only to skip unrelated docs, NetworkX, middle-version REST, graphviz, Neo4j, build, DGL, Spark, and AI/UMAP fanout for schema-only PRs.
• Keeps schema-artifacts, test-gfql-core, test-pandas-compat-gfql, test-core-python, test-polars, and changed-line-coverage for schema runtime/artifact edits.
• Keeps docs/source schema edits out of schema_only so docs-only schema changes run docs validation, and mixed schema code+docs fail broad.

Safety checks

• git diff --check
• Extracted changes-job shell block with bash -n
• PyYAML parse of .github/workflows/ci.yml: 32 jobs
• Credential/token scans clean
• Source-derived scenario evaluator covering schema.py, schema artifacts, public schema test, schema docs, mixed schema code+docs, PlotterBase/shared surfaces, GFQL schema_effects, and workflow infra

Coverage notes

Schema-only still runs changed-line-coverage and explicitly keeps test-core-python/test-polars so coverage artifacts remain available. test-pandas-compat-gfql now includes graphistry/tests/compute/gfql/test_public_schema.py and runs for schema_lane to preserve pandas-version coverage for public schema behavior.

[lmeyerov]

Closing after broader PR-history audit: this schema-only slice is safe but not historically ROI-supported right now. Last 500 merged PRs had no non-changelog schema_only hits, so we should hold off until a real schema PR/run shows over-fanout.