Module mutations 

Mutation backend abstraction.

This module defines MutationBackend, a trait that abstracts every Things 3 mutation operation behind a single interface. The MCP server holds an Arc<dyn MutationBackend> and dispatches all writes through it. This unblocks issue #120’s migration from direct SQLite writes (which CulturedCode warns can corrupt the user’s database) to AppleScript-based mutations.

Two implementations are planned:

• SqlxBackend — wraps the existing direct-DB writes on crate::ThingsDatabase. Today’s behavior; useful for offline tests and CI.
• AppleScriptBackend — to be added in #124. The default in production after #125.

Why #[async_trait] instead of native async fn in traits

The trait must be object-safe so the server can hold Arc<dyn MutationBackend> and choose between backends at runtime. Native async-fn-in-trait (Rust 1.75+) requires #[trait_variant] shims for dyn dispatch and produces unnameable opaque return types — too much friction for marginal benefit. #[async_trait] boxes the future, which is exactly what dyn needs.

Structs

SqlxBackend

Traits

MutationBackend

Abstraction over every Things 3 mutation operation exposed as an MCP tool.