[lmeyerov]

Summary

Stage 2 of #2249. Collected nodes keep their identity through WITH collect(node) ... MATCH ... WHERE candidate IN nodes re-entry on pandas, Polars and cuDF. Grouped whole-entity keys (RETURN b, count(*), RETURN rel, count(*)) also stay identity-distinct instead of collapsing to null.

• Row-token re-entry lifecycle (Stage 1) plus native collected-node identity membership on pandas and Polars.
• Alias and property provenance repair. Ordinary named-node Polars paths keep their baseline result width.
• Typed null alias-marker columns via Engine.null_column: Polars and cuDF are dtype-preserving, and pandas keeps the object None sentinel for parity.
• entity_props._false_mask builds its all-False mask from isna(). The old (base == True) & False raised ArrowTypeError on cuDF integer id columns.

Rebased onto master 5819f46b after #2258 landed. The duplicate #2258 cherry-pick is gone.

Evidence at head b9dc6b6b

• Hosted CI run 41293025708: 93/93 checks green, including tck-gfql (4289 passed, 0 failed) and all 13 gfql-routes-off modes with empty divergence ledgers. RTD is green.
• GPU gate in graphistry/test-rapids-official:26.02-gfql (cuDF 26.02) with an identity-asserted runner. The set is all 100 cuDF and Polars-GPU ids collected from the 11 touched test files with TEST_CUDF=1 TEST_POLARS_GPU=1. Skips count as failures. Result: 98 passed, 2 failed, 0 skipped.
  • Both failures are inherited. They fail the same way on master 5819f46b in the same image:
    • test_row_pipeline_order_by_stringified_list_subscript_expression_on_cudf_when_available
    • test_row_pipeline_cudf_ast_map_literal_vector_values_when_available
  • They are tracked in GFQL Cypher cuDF: 6 test_lowering cuDF cases fail on master (list/dynamic expressions raise host-conversion TypeError; cugraph write flips edge orientation) #2269, alongside the other cuDF list-expression cases that hit the implicit __array__ conversion.
• New regression test test_append_property_segments_integer_alias_typed_false_mask[pandas,cudf] passes on both engines.

Follow-ups

• GFQL Cypher cuDF: 6 test_lowering cuDF cases fail on master (list/dynamic expressions raise host-conversion TypeError; cugraph write flips edge orientation) #2269: inherited cuDF list-expression failures.
• GFQL: unify engine-specific empty-frame / row-slice idioms behind engine-agnostic helpers #2270: engine idiom helpers in Engine.py.
• pyg-bench branch work/2249-polars-alias-width-pin: Polars named-node width pin, to open as its own PR.

Part of #2249.

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