Make Proof construct interpreters from relation values

[yoohaemin]

Alternative to #387: make Proof construct a ReifiedRelation directly through apply(relation). Composition passes the actual child relation values to child proofs, and syntax constructs and retains the interpreter before execution. This removes the conversion-function layer and the intermediate bound-proof wrapper.

Datasource and generator registrations retain their cached interpreters. This changes the custom-proof API: implementations define apply(relation) instead of parameterless reify; cardinality-specific contramap methods take the source relation explicitly, and integration adaptation helpers accept an already constructed interpreter.

Validation: the full JVM/Scala.js test matrix, fmtCheck, docs/mdoc, and githubWorkflowCheck passed. Additional constructor-reuse and source-binding regressions passed on Scala 2.13 and 3.3, on JVM and Scala.js. Documentation reports six existing link warnings.