Infer a common error type for composed relation proofs

[yoohaemin]

Composed and zipped relation proofs need a shared error type that accepts failures from either child. Resolve the child proofs with independent error types, then infer the parent error type through CommonError and widen the interpreters using its subtype evidence.

Relation-level regression tests materialize single, optional, many, and zipped proofs with distinct left/right failure types. They check shared-type inference and preservation of both failures in single and batched execution, as well as successful outputs, absent optional results, and empty collections.

Validation: core tests passed on Scala 2.13 and 3.3, on JVM and Scala.js, together with fmtCheck. Proof construction is reviewed separately in #387, stacked on this branch.

[Copilot]

🟡 Changes recommended

Relation-level tests do not cover composed proofs with independent child error types.

1 open finding

• Add regression coverage for preserving distinct child errors in composed proofs · New

What changed in this PR

Introduces shared error-type inference for composed bifunctor relation proofs.

Changes:

• Adds CommonError subtype evidence.
• Widens child interpreters to a shared error type.
• Adds evidence-level regression tests.

File	Description
CommonError.scala	Defines common-error evidence.
bifunctor/​proof.scala	Applies shared errors across composition forms.
CommonErrorSpec.scala	Tests evidence inference and value preservation.

🧠 Review effort: Balanced

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[Copilot]

🟢 Approval recommended

The implementation consistently handles all composition variants with comprehensive cross-platform regression coverage.

0 open findings

1 resolved since last review

• Add regression coverage for preserving distinct child errors in composed proofs

🧠 Review effort: Balanced