//! Protobuf wire format mirroring the kernel's plan / schema / expression IR.
//!
//! Each submodule wraps the prost-generated code for one `.proto` file in
//! `kernel/proto/`. Consumers can serialise a kernel-built plan and ship it
//! across a process / language boundary (Rust kernel -> JVM engine, etc.).
//!
//! - [`schema`] -- mirror of `kernel/src/schema/mod.rs` (`DataType`, `PrimitiveType`, `StructType`,
//! ...).
//! - [`expressions`] -- mirror of `kernel/src/expressions/mod.rs` and `scalars.rs` (`Expression`,
//! `Predicate`, `Scalar`, `ColumnName`, ...).
//! - [`plan`] -- mirror of `kernel/src/plans/ir/{plan,nodes}.rs` (`Plan`, `PlanNode`, `Operator`,
//! `RefId`, per-variant payload messages).