feat(hql): Intersect function

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Greptile Overview

Greptile Summary

This PR adds a new INTERSECT traversal step to HelixQL end-to-end: Pest grammar support, parser StepType::Intersect, analyzer validation and codegen, plus a new engine adapter (IntersectAdapter) and multiple test fixtures (engine-level Rust tests and HQL tests).

At runtime, the engine implementation collects upstream items, executes a provided sub-traversal closure per item to get candidate results, then returns only values whose IDs appear in every sub-result set (set intersection semantics). The generator emits .intersect(|val, db, txn, arena| { G::from_iter(..) <substeps> ... }) to wire the HQL syntax to the engine adapter.

Main issue to address before merge: one existing HQL test (user_test_12) changes the meaning of GetIndicatorsWithTimeParams from “has any matching time parameter” to “connected to all matching time parameters provided”, which will change expected results when multiple time_vals are passed and should be clarified/adjusted.

Important Files Changed

Filename	Overview
helix-db/src/grammar.pest	Adds INTERSECT(_::<traversal>) as a new util step in the Pest grammar.
helix-db/src/helix_engine/tests/traversal_tests/intersect_tests.rs	Adds engine-level Rust tests for IntersectAdapter across several graph scenarios.
helix-db/src/helix_engine/tests/traversal_tests/mod.rs	Registers the new intersect test module.
helix-db/src/helix_engine/traversal_core/ops/util/intersect.rs	Introduces IntersectAdapter for RoTraversalIterator; currently collects upstream and sub-traversal results and intersects by id.
helix-db/src/helix_engine/traversal_core/ops/util/mod.rs	Exports the new util::intersect module.
helix-db/src/helixc/analyzer/methods/traversal_validation.rs	Adds analyzer support for StepType::Intersect by inferring sub-traversal type and emitting generator step.
helix-db/src/helixc/generator/traversal_steps.rs	Adds generator Step::Intersect and emits intersect(...) closure code (currently drops sub-traversal errors).
helix-db/src/helixc/generator/utils.rs	Imports IntersectAdapter into generated code prelude.
helix-db/src/helixc/parser/graph_step_parse_methods.rs	Parses intersect_step into StepType::Intersect using parse_expression.
helix-db/src/helixc/parser/types.rs	Adds StepType::Intersect and updates PartialEq accordingly.
hql-tests/tests/intersect/helix.toml	Adds new HQL test project config for intersect feature.
hql-tests/tests/intersect/queries.hx	Adds HQL queries that use INTERSECT to find articles by all tags.
hql-tests/tests/intersect/schema.hx	Defines Tag/Article/HasTag schema for intersect tests.
hql-tests/tests/search_v_with_embed/queries.hx	Updates test query to take item param and pass it to Embed().
hql-tests/tests/user_test_12/queries.hx	Replaces EXISTS-based semantics with INTERSECT-based query; changes meaning from 'any matching time parameter' to 'all matching time parameters'.
hql-tests/tests/user_test_12/queries.rs	Adds generated Rust code for user_test_12; includes new intersect usage and embedded schema/config.

Sequence Diagram

sequenceDiagram
    participant User as HQL User
    participant Parser as helixc parser (pest)
    participant Analyzer as helixc analyzer
    participant Gen as helixc generator
    participant Engine as traversal engine

    User->>Parser: Parse traversal with ::INTERSECT(_::<sub-traversal>)
    Parser->>Analyzer: Emit StepType::Intersect(Expression)
    Analyzer->>Analyzer: infer_expr_type(expr, Some(cur_ty))
    Analyzer->>Gen: GenerateStep::Intersect { traversal }
    Gen->>Engine: Emit Rust: .intersect(|val, db, txn, arena| G::from_iter(..).<substeps>...)
    Engine->>Engine: Collect upstream values
    loop for each upstream value
        Engine->>Engine: Run sub-traversal closure => Vec<TraversalValue>
    end
    Engine->>Engine: Intersect result sets by TraversalValue.id()
    Engine-->>User: Iterator of intersected TraversalValue results

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Additional Comments (1)

hql-tests/tests/user_test_12/queries.hx
Semantic change in test

GetIndicatorsWithTimeParams was changed from EXISTS(_::Out<HasTimeParameter>::WHERE(...)) ("indicator has at least one matching time parameter") to N<TimeParameter>::...::INTERSECT(_::In<HasTimeParameter>) ("indicator is connected to all matching time parameters in time_vals"). This will produce different results whenever time_vals has >1 entry (e.g., an indicator linked to only one of them used to match, now it won’t). If the intent is to preserve the old behavior, this should stay as EXISTS; if the intent is a behavior change, the test name/fixture should be updated to reflect the new contract.