[tmm]

Typed named queries and mutators for the Rust client, as tmm decided (option A): define_query and define_mutator behind a new optional serde feature, shaped like the Swift client's defineQuery and defineMutator. The context comes from the Zero instance. Stacked on #206; it rides with 0.2.0 (minor changeset).

const LABELS: QueryDefinition<ProjectArgs, Context, Label> =
    define_query("labels", |args, ctx| labels_query(&args.project_name));
const CREATE_ISSUE: MutatorDefinition<CreateIssueArgs, Context> =
    define_mutator("issue.create", |tx, args, ctx| { ... });

let view = zero.materialize(&LABELS.with(ProjectArgs { project_name }), &ttl)?;
zero.mutate_request(CREATE_ISSUE.with(args));
Zero::new(schema, context, vec![CREATE_ISSUE.mutator()], ...);

• zero_client::define: define_query → QueryDefinition, .with(args) → QueryRequest (Send, cloneable, read(view)); define_mutator → MutatorDefinition, .with(args) → MutateRequest, .mutator() to register. NamedQuery<C> and NamedMutator<C> erase the types for lists of definitions (registration, drift tests). An argument type with no fields, such as (), sends no arguments, as JS's undefined does.
• query::ZeroQuery<C>: materialize, run, and preload take an AnyQuery, a typed Query, or a QueryRequest (unsized too, so &dyn ZeroQuery<C> works). Existing calls compile unchanged.
• client::MutateRequest { name, args } and Zero::mutate_request(request). Naming: JS and Swift both call this zero.mutate(request). Rust has no overloading and mutate(name, args) already exists, so mutate keeps its signature and the request form is mutate_request, matching Swift's MutateRequest type name.
• json::Value and Object implement serde; json::to_value and json::from_value.
• The default build still has no third-party dependencies (cargo tree -p zero-client -e normal lists only zero-client); serde is default-features = false, features = ["std"].
• zbugs-model, Rune, and GPUI now use the definitions; zbugs-model's own NamedQuery, MutatorDef, MutateRequest, AnyNamedQuery, AnyMutator, and serde_json bridge are gone. PARITY.md's defineQuery and defineMutator rows are updated.

No change to the wire protocol, query or mutator names and arguments, or storage: zbugs-model's drift test (the same vectors zbugs-swift checks) passes unchanged in content, and I checked that it still fails when a query or mutator is renamed.

Checked locally: cargo clippy --workspace --all-targets --all-features -D warnings, cargo test --workspace --all-features, cargo doc -D warnings; zbugs-model cargo clippy -D warnings and cargo test (drift); zbugs-rune and zbugs-gpui (and --features e2e) cargo clippy -D warnings and cargo test on Linux.