feat(gfql): add encode call parity - #1584

Summary

• Add GFQL call() safelist/runtime parity for encode_edge_icon and encode_axis.
• Add typed call() params/overloads for both entry points.
• Strengthen encode validators for palette string entries and categorical_mapping key/value/default types with anchored diagnostics.
• Keep execution on the existing generic Plottable call dispatch path.

Closes #1241

New encode entry points

• encode_edge_icon: maps edge column values to icon encodings through GFQL call(), including categorical_mapping/default_mapping/as_text validation.
• encode_axis: applies axis rows through GFQL call(), mirroring direct Plottable encode_axis rows behavior.

Validator-error samples

• encode_edge_icon: Invalid value for parameter 'categorical_mapping' in 'encode_edge_icon': categorical_mapping['email'] must be a string
• encode_axis: Invalid value for parameter 'rows' in 'encode_axis': rows[1] must be a dictionary
• encode_point_color: Invalid value for parameter 'palette' in 'encode_point_color': palette[1] must be a string
• encode_point_size: Invalid value for parameter 'default_mapping' in 'encode_point_size': default_mapping must be a number

LOC buckets

• Production: +133 / -13, net +120 LOC
• Tests: +286 / -0, net +286 LOC
• Changelog/docs: +1 / -0, net +1 LOC

Compiler-plan surface touched

No. This is GFQL public call validation/type parity plus generic Plottable method dispatch; no logical/physical planning semantics, route names, dispatch contracts, diagnostics, source spans, pass manager, schema/type-system, IR metadata, remote schema, or compatibility executor surfaces were changed.

Validation

• python3 -m pytest -q graphistry/tests/compute/test_gfql_call_validation.py -k "Encode" -> 21 passed, 31 deselected
• python3 -m pytest -q graphistry/tests/compute/test_call_operations.py -k "encode" -> 2 passed, 12 deselected
• ./bin/ruff.sh graphistry/compute/gfql/call/validation.py graphistry/models/gfql/types/call.py graphistry/compute/ast.py graphistry/tests/compute/test_gfql_call_validation.py graphistry/tests/compute/test_call_operations.py -> passed
• ./bin/typecheck.sh graphistry/compute/gfql/call/validation.py graphistry/models/gfql/types/call.py graphistry/compute/ast.py -> passed
• python3 -m pytest -q graphistry/tests/ -k "encode" -> 51 passed, 6 skipped, 4982 deselected
• python3 bin/changed_line_coverage.py --data-file .coverage --base-ref origin/master --head-ref HEAD --min-percent 80 --output-dir /tmp/clc-local-1584-rebased -> passed, 69/69 changed executable lines covered in focused local reproduction