GFQL deletion: shrink row-pipeline string-expression fallback for map/list AST values #1469

Summary

The row-pipeline expression evaluator has an AST path, but it still drops back to the legacy string evaluator for some structured/vector values. A concrete deletion target is to make the AST path cover map/list vector values and then shrink _gfql_eval_string_expr fallback usage.

Current evidence

On current origin/master:

• graphistry/compute/gfql/row/pipeline.py::_gfql_eval_expr_ast() returns (False, None) for MapLiteral values where the evaluated value has .astype, with the comment: Vector map values are deferred to legacy evaluator for now.
• _gfql_eval_string_expr() remains the fallback bridge and is called from projection/order/explode paths.
• Existing tests compare AST output to legacy_out = ctx._gfql_eval_string_expr(...).

Goal

Promote the remaining safe structured expression cases to the AST evaluator and delete the corresponding legacy string-eval fallback branch/tests.

Scope

• Add pandas/cuDF coverage for map/list literals with vector-valued entries where the fallback is currently used.
• Implement AST evaluator handling for those values, preserving null semantics.
• Remove the Vector map values are deferred to legacy evaluator for now fallback when covered.
• Narrow _gfql_eval_string_expr() callers or add explicit unsupported errors where AST cannot cover the shape.

Out of scope

• Full expression engine rewrite.
• Public API changes.
• Non-GFQL row pipeline work.

Acceptance

• The targeted map/list vector cases no longer call _gfql_eval_string_expr().
• Legacy comparison tests are rewritten to assert AST-native behavior instead of comparing to the legacy evaluator.
• Focused pandas tests pass; cuDF coverage runs where applicable.
• CI and relevant DGX GFQL/cuDF validation pass if cuDF path changes.

Related

• [META] GFQL deletion wave: remove 2K+ LOC of obsolete compatibility paths #1419
• GFQL compat-executor reachability shrink #1467