[tmm]

tmm's decision 2(b): Swift's mutation-outcome listener, ported to Rust.

let remove = zero.add_mutation_outcome_listener(|o: &MutationServerOutcome| {
    if let MutationOutcome::AppError { message, .. } = &o.outcome { show(&o.name, message) }
});

A mutation's server result settles with ZeroError when the client gives up connecting (disconnect_timeout_ms), yet the mutation is replayed after a reconnect. This listener hears its real outcome then, once per mutation, so an app has one place to show rejections. JS has no counterpart; this is Swift's addMutationOutcomeListener.

• Which mutations: the ones this client ran. Rebased mutations, and mutations replayed from storage on launch, aren't reported, as in Swift.
• When: the first server outcome (pushResponse, a mutationsPatch error, or lmid advancing). The server result settles first.
• Lifetime: listeners live on Zero, so they survive an in-place client replacement.
• Cost with no listeners: one map entry with the name per mutation. Args are moved, not copied.

Tests: tests/mutation_outcomes.rs, 4 tests:

• each outcome is heard once;
• a mutation replayed after the disconnect timeout is heard;
• unsubscribe stops it;
• listeners outlive a replaced client.

Each fails against a plausible wrong version: no dedupe, reporting from server, or attaching to the first engine only.

Benchmarks (mutations and pokes, 2 passes × 5 alternating rounds vs main): all within ±3.2%, with overlapping ranges.

Changeset: zero-rust minor (with 0.3.0).

Checked locally: clippy -D warnings, tests, docs, and conformance (66,384 cases, 0 failures).