Cross-port codegen audit: subtype-proxy persistence gating in Java/Python/Kotlin/C# generators #251

Follow-up from #248 (PR #249), design spec §9b.

#248 fixed the TypeScript migrate-ts + codegen-ts fail-open where "is this object a DB thing?" was decided by an object-subtype compare instead of by declared/inherited source.* presence. The Java (codegen-spring), Python, Kotlin, and C# generators emit repositories/controllers/DTOs — audit whether their emission gates are the same subtype-proxy with a sourceless fail-open.

Schema/DDL is TS-owned (ADR-0015), so no other port emits CREATE TABLE — lower stakes than the TS half — but the durable principle is the same: DB participation derives from source.*, never from a subtype name. A sourceless custom object.* subtype (or a plain sourceless entity) should not get a generated repository/controller.

Same family as the #228 out-of-scope cross-port audit list. Ref: §9b.