test(gfql): viz-filter-pipeline acceptance suite + regression benchmark (L3)

CHANGELOG.md

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 ### Added
+- **GFQL viz-filter-pipeline acceptance suite + regression benchmark (viz-filter L3)**: `test_viz_pipeline_conformance.py` — curated full-panel pipelines (node+edge filters, exclusion-dominates composition, `(pred OR IS NULL)` keep-null leaves, both EXISTS prune-isolated flavors, searchAny composition, deterministic paging), graph-state prune shapes with exact node+edge pins, a 40-seed panel-state fuzzer with an independent plain-pandas second oracle, and a case/regex/unicode trick matrix (ß/İ full-case-mapping pins, metachar literal-vs-regex, null cells, Categorical) — all parity-or-NIE across pandas/cuDF/polars/polars-gpu. `benchmarks/gfql/viz_filter_pipeline.py` — six streamgl-viz panel scenarios (filters, keep-self GRAPH prune, EXISTS prune, node/edge search, combined) at 100K/1M/10M with native-frame-per-engine fairness, an NIE-tolerant matrix, and JSON receipts (first receipt: 100k × 4 engines, everything within the ~350ms interactive reference except pandas combined). Documented findings from the first runs: the same-path WHERE route dedupes parallel edges (diverges from the panel algebra's edge multiplicity — pinned + tracked), and edge-alias searchAny declines on polars (tracked).
 - **GFQL Cypher `searchAny(entity, term[, opts])` cross-column search predicate + `g.search_nodes()`/`g.search_edges()` (viz-filter L2, native on all four engines)**: True where ANY of the entity's columns matches the term — the streamgl-viz inspector's table-search semantics as a composable WHERE predicate: OR across columns; case-insensitive substring by DEFAULT (case-folded, never regex — the common call avoids every engine regex limit); regex opt-in obeying the same per-engine decline rules as `=~`; dtype gate AS SEMANTICS (string columns always; integer columns iff the term is a numeric literal, per the inspector's `/^[0-9.-]+$/` gate; floats/dates/booleans reachable via the explicit `columns:` list). Options map `{caseSensitive, regex, columns}` is strict-validated (unknown keys error listing the valid ones); unbound aliases and missing explicit columns error clearly; null cells never match. Lowered like the pattern-predicate markers (a `search_any` row op + fresh marker column), so it composes through AND/OR/NOT and different node/edge terms coexist in one pipeline. Per-column matching reuses the parity-hardened `Contains` predicate on pandas/cuDF and a lowercase-fold/any_horizontal lowering on polars; oracle-pinned + 4-engine parity-or-NIE conformance cases. Python twins `g.search_nodes(term, columns=, case_sensitive=, regex=)` / `g.search_edges(...)` filter their own table and return a Plottable (polars-frame twins decline honestly for now — use the cypher op). Honest declines (NIE, use engine='pandas'): edge-alias searchAny on polars, and explicit columns beyond string/int/bool dtypes on polars AND cuDF (incl. floats: repr diverges across engines — dgx-probed).
 - **GFQL Cypher `EXISTS { <pattern> }` pattern-existence subqueries (openCypher-standard), native on all four engines**: `WHERE EXISTS { (n)-[:R]->() }` and `WHERE NOT EXISTS { (n)--() }` now parse and run — the declarative prune-isolated building blocks for the streamgl-viz filter pipeline. An `EXISTS` body reuses the existing pattern-predicate lowering wholesale (`semi_apply_mark` / `anti_semi_apply` row ops), so pandas/cuDF worked immediately; the polars engine gains NATIVE lowerings for the semi-apply family (correlated key sets computed by the polars chain executor's named-flag columns; order-preserving `is_in` joins) plus `rows(binding_ops=...)` for the single-entity row table — previously all honest-NIE. Aliases introduced inside the braces are existentially scoped (`EXISTS { (n)--(m) }` allowed with `m` unbound outside — bare pattern predicates keep the conservative guard), inline property maps work, and the one supported inner `WHERE` form is endpoint inequality — `EXISTS { (n)--(m) WHERE m <> n }`, the drop-self-loop prune-isolated flavor (pandas/cuDF filter the correlated bindings; polars excludes self-loop edges, which is exactly the `m <> n` witness). Both prune flavors are oracle-pinned in the conformance matrix on a self-loop discriminator graph, 4-engine parity-or-NIE. Honest declines with clear errors: `EXISTS` in RETURN/WITH projections, general inner `WHERE`, multi-pattern bodies, full `MATCH..RETURN` subquery bodies, multi-alias correlation on polars.
 - **GFQL polars execution config is Python-settable and live**: `set_cpu_streaming(bool)` and `set_gpu_executor('in-memory'|'streaming')` in `graphistry.compute.gfql.lazy` (plus the public `GPU_EXECUTORS` options and `GpuExecutor` type) set the CPU-streaming / GPU-executor knobs from Python. They resolve **Python override > environment variable > default**, read **live** per collect — previously these were env-only (`GFQL_POLARS_CPU_STREAMING` / `GFQL_POLARS_GPU_EXECUTOR`) and frozen at import, so neither a Python setting nor a post-import env change took effect. `None` resets a setter to env/default.