GFQL native types T4: Arrow/type bridge contracts and coercion semantics

[lmeyerov]

Summary

Execution slice T4 under #1262 / #1046.

Define and implement the Arrow/type bridge surface for GFQL native types, including explicit coercion/confidence semantics at export/import boundaries.

Scope

In scope:

• Establish contract for to_arrow / from_arrow typed schema interchange points
• Define coercion behavior (widening/narrowing/nullability interactions) and confidence metadata semantics
• Add regression tests for representative node/edge/property type combinations across pandas/cudf pathways where applicable

Out of scope:

• Strict-mode binder rule changes (T2)
• Core metadata propagation mechanics (T3/T3.b)

Acceptance

1. Arrow bridge contract is documented in code/docs with deterministic semantics.
2. Round-trip and boundary tests cover coercion + nullability behavior.
3. No regressions in existing GFQL/Cypher execution behavior.
4. CHANGELOG entry references Meta: #1046 native type-system staged decomposition program #1262 and [FEA] GFQL native type system: schemas, inference, validation, and Arrow representation #1046.

Related

• Parent: Meta: #1046 native type-system staged decomposition program #1262
• Umbrella: [FEA] GFQL native type system: schemas, inference, validation, and Arrow representation #1046
• Prior lanes: GFQL native types T2: binder-time schema validation in strict mode #1302, GFQL native types T3: propagate nullability/type metadata through bound/logical plan seams #1300, GFQL native types T3.b: migrate inline nullable patterns onto ir.metadata helpers #1309
• DAG: Meta: GFQL compiler/planner post-M4 execution DAG (active wave) #1259