engine: protobuf ProjectPatch/ModelPatch/ModelOperation messages are dead schema #538

Problem

src/simlin-engine/src/project_io.proto (lines 402-459) defines a family of patch messages:

• ProjectPatch { project_ops, models }
• ModelPatch { name, ops }
• ProjectOperation (oneof: SetSimSpecsOp, SetSourceOp)
• ModelOperation (oneof: UpsertStockOp, UpsertFlowOp, UpsertAuxOp, UpsertModuleOp, DeleteVariableOp, RenameVariableOp, UpsertViewOp, DeleteViewOp, UpdateStockFlowsOp)
• the associated *Op payload messages (UpsertModuleOp, etc.)

These compile to prost types in src/simlin-engine/src/project_io.gen.rs (ProjectPatch at line 819, ModelOperation at 848, UpsertModuleOp at 908).

No Rust code references the prost-generated patch types. A repo-wide grep for project_io::ProjectPatch / project_io::ModelPatch / project_io::ModelOperation / project_io::UpsertModuleOp returns zero matches, and src/simlin-engine/src/serde.rs has no conversion functions for them.

The actually-exercised patch path is JSON-only and uses a separate, hand-written native type that is unrelated to protobuf:

• src/simlin-engine/src/patch.rs defines a native ProjectPatch / ModelPatch / ModelOperation (plain Rust structs/enums over datamodel::*, not prost-derived).
• src/libsimlin/src/patch.rs defines JsonProjectPatch / JsonModelOperation (serde) and converts them to the native engine::ProjectPatch via convert_json_project_patch / convert_json_model_operation, called from the simlin_project_apply_patch FFI entry point.
• src/simlin-mcp-core/src/tools/edit_model.rs likewise builds the native simlin_engine::ProjectPatch directly.

So there are two same-named-but-unrelated ProjectPatch families, and the protobuf one is dead: it has a generated Rust type but no conversion path and no caller.

Why it matters

• Maintainability / latent inconsistency: a contributor extending the patch surface (e.g. for Vensim macro support, where this was discovered) reasonably assumes project_io.proto is the source of truth and may add proto messages that are never wired up — or may not realize the proto definitions exist and are stale.
• Protobuf versioning discipline: the repo treats project_io.proto as a versioned wire format (there is a DB of serialized instances). Carrying dead patch messages in that file muddies which messages are actually load-bearing.
• It is either (a) genuinely dead schema that should be deleted, or (b) an intended-but-never-implemented proto patch path. Either way the current state is undocumented and confusing.

Component(s) affected

• src/simlin-engine/src/project_io.proto
• src/simlin-engine/src/project_io.gen.rs (generated)
• (contrast with the live path: src/simlin-engine/src/patch.rs, src/libsimlin/src/patch.rs, src/simlin-mcp-core/src/tools/edit_model.rs)

Possible approaches

1. Delete the dead messages from project_io.proto (ProjectPatch, ModelPatch, ProjectOperation, ModelOperation, and the *Op payload messages that are not used elsewhere) and regenerate. Since these messages are never serialized to the DB (only Project is), removing them is wire-safe. Verify each *Op's inner payload message isn't shared with a live message before deleting.
2. Or decide proto is meant to be the patch wire format, and implement the project_io <-> patch.rs conversion in serde.rs plus a proto FFI entry, retiring the bespoke JsonProjectPatch in libsimlin. This is the larger change and should only happen if there's a real need for a binary patch format.

Given the JSON path is the one with full FFI + MCP coverage, option 1 (delete) is the likely answer, but the choice should be made deliberately.

Discovery context

Identified during a codebase investigation for Vensim macro support; out of scope for that work.