[yoohaemin]

Zipped relations now combine their two branches through one internal zipWith helper that delegates to foreach. foreach remains the only overridable operation for combining independent effects, so bulk traversal and zipped relations cannot select conflicting batching implementations.

Both interpreter models use the helper for single and bulk zips. It is package-private and final; its internal casts rely on foreach preserving result count and positional correspondence. The default foreach sequences directly with flatMap and map, avoiding recursion through the helper. Fetch and ZQuery keep their existing foreach implementations; no separate zip override is required.

The Fetch regression exercises actual zipped relations backed by a shared datasource and checks that independent keys share one batch. The asynchronous assertion is returned to the Scala.js test runner.

Parent of #400. Review and merge this PR first; #400 builds Kyo query plans through the same foreach extension point.

Validation: core, ZQuery, and Fetch suites on Scala 2.13 and 3.3.9, plus Fetch Scala.js tests. Changed Scala sources formatted.

[Copilot]

🟡 Changes recommended

The shared Fetch regression uses JVM-only unsafeRunSync, breaking Scala.js test compilation.

1 open issue

• Use cross-platform async execution in shared Scala.js test · New

What changed in this PR

Replaces erased zip traversal with a typed interpreter primitive, preserving specialized batching behavior.

Changes:

• Uses typed zipWith for single and bulk relations in both interpreter models.
• Adds batching overrides for ZQuery and Fetch.
• Adds a Fetch batching regression and test dependencies.

File	Description
zquery/​src/​main/​scala/​decrel/​reify/​zquery.scala	Uses batched zip composition.
fetch/​src/​test/​scala/​decrel/​reify/​FetchZipSpec.scala	Adds a shared-datasource batching regression.
fetch/​src/​main/​scala/​decrel/​reify/​fetch.scala	Implements zip using applicative mapN.
core/​src/​main/​scala/​decrel/​reify/​monofunctor/​reifiedRelation.scala	Replaces erased traversal with typed zip.
core/​src/​main/​scala/​decrel/​reify/​monofunctor/​access.scala	Exposes the protected zip primitive.
core/​src/​main/​scala/​decrel/​reify/​bifunctor/​reifiedRelation.scala	Replaces erased traversal with typed zip.
core/​src/​main/​scala/​decrel/​reify/​bifunctor/​access.scala	Exposes the protected zip primitive.
build.sbt	Adds Fetch test dependencies.

_{Review effort: Balanced}