[yoohaemin]

Single <>: placed the left relation in both branches of a zip, so an uncached effect could execute it twice and retain a different value from the one passed downstream. Compose the left relation once, then zip Self with the downstream relation, matching Optional and Many.

Tests in both interpreter models use a deferred counter to verify single and bulk coherence and repeated execution. Existing relation-shape tests retain their public output-type assertions without depending on the old duplicated AST.

Validation: core/test, core3/test, coreJS/test, coreJS3/test, fmt. All passed.

[Copilot]

🟡 Changes recommended

The new inner zip conflicts with interpreter proof bounds when the downstream relation accepts a strict supertype of the retained value.

1 open issue

• Inner zip reification fails for permitted subtype-to-supertype inputs · New

What changed in this PR

Fixes retained Single composition so the left relation runs once and retains the value passed downstream.

Changes:

• Composes the left relation before zipping Self with the downstream relation.
• Adds coherence tests for both interpreter models.
• Preserves public output-type assertions without fixing the internal composition shape.

File	Description
core/​src/​test/​scala/​decrel/​RelationSpec.scala	Removes old composition-shape assertions.
core/​src/​test/​scala/​decrel/​reify/​monofunctor/​KeepCoherenceSpec.scala	Tests single, repeated, and bulk execution coherence.
core/​src/​test/​scala/​decrel/​reify/​bifunctor/​KeepCoherenceSpec.scala	Adds equivalent bifunctor coherence tests.
core/​src/​main/​scala/​decrel/​syntax.scala	Evaluates retained Single relations once.

_{Review effort: Balanced}