Row iteration must support terminating early without throwing

Cluster: streaming

Scenario

A developer iterates a worksheet's rows to validate uploaded data. On the first invalid row they want to stop immediately and report the error, not visit every remaining row. With a forEach-style callback iteratee, the legacy shape, there is no supported early break: returning from the callback ends only that one invocation. The workaround people fall back on is throwing an exception and catching it around the whole loop, which conflates control flow with real errors and, in async contexts, risks unhandled rejections.

Spec note, not a corpus case: this is an API-ergonomics gap with no malformed output to assert, since the current iteration produces correct rows and just cannot be stopped cleanly. The durable value is the desired iteration API and its stop semantics.

Desired behavior

• The row and cell iteration API offers a first-class way to terminate early, so validate-and-bail patterns are ergonomic and never require exceptions for control flow.
• Prefer real (async) iterables. Exposing rows as Iterable and AsyncIterable lets standard for (const row of ...) { if (bad) break; } and for await (const row of ...) work naturally, so break and early return compose with the enclosing function, and async iteration covers streamed reads of large files.
• If a callback iteratee is kept, give it explicit stop semantics, for example returning false halts iteration in the style of Array.prototype.some and every, and document it. Silent "return does nothing" is the trap to remove.
• Early-termination behavior is consistent between the in-memory worksheet model and the streaming reader.

Open questions

• Keep a callback iteratee at all, or standardize solely on (async) iterators?
• If both exist, what is the stop signal for the callback form: return false, or a passed-in stop handle?
• On the streaming reader, can breaking out tear the underlying stream down promptly, without leaking file handles or leaving the parser mid-state?
• Does includeEmpty, which skips empty rows, change the row numbers and indices exposed to the consumer during iteration?

Related: streaming-read-emits-all-worksheets, row-iteration-includes-trailing-empty-cells, streaming-writer-row-commit-backpressure.