The streaming writer's worksheet type is a narrowed, forward-only reflection

Cluster: types

Scenario

A TypeScript caller opens a streaming, forward-only workbook writer and adds a worksheet, expecting the same rich worksheet the buffered API returns. The published types say so, typing the streaming worksheet as the full random-access worksheet, but at runtime it is a narrower, forward-only object. Methods the type promises (insertRow, and other random-access, splice and back-reference operations) either do not exist or throw once earlier rows have been flushed. The type lies, so the compiler green-lights code that fails at runtime.

The same drift appears at the workbook level: the streaming writer is typed as if it extended the buffered workbook, but it does not. It is a distinct forward-only surface with its own, smaller set of capabilities.

Spec note, not a corpus case: the defect is a type-surface fidelity gap on a forward-only object, pinned by type-level tests, not a malformed-output bug reproducible from a data file.

Desired behavior

• The streaming worksheet has its own precise type that exposes only what a forward-only writer can honor. Random-access operations that require rewriting already-flushed rows (inserting or splicing a row above the write frontier, re-reading a committed cell, arbitrary back-reference) are absent from the type, so misuse is a compile error rather than a runtime surprise.
• The streaming workbook writer is typed as a distinct surface, not as a subtype of the buffered workbook. Shared capabilities may come from a common minimal interface, but the writer never claims operations it cannot perform on a forward-only stream.
• Commit and forward-only semantics are visible in the types: adding rows and cells and committing a worksheet or the workbook are the vocabulary, and the forward-only constraint is expressed by omission, so the ergonomic loop (stream rows, commit, move on) is exactly what the types make easy.
• Where a buffered-only operation has a streaming analogue with different mechanics, the analogue is named honestly rather than aliased to the buffered method it does not fully implement.

Open questions

• The exact minimal interface shared between buffered and streaming worksheets, against what stays buffered-only, driven by the unification question in unified-streaming-and-buffered-io.
• Whether any "insert above the frontier" operation is supportable at all on a forward-only writer, via a buffer-until-commit window, or is categorically out of scope.
• Naming for the streaming worksheet and writer types so the forward-only nature is discoverable at the call site, not just in prose.

Related: unified-streaming-and-buffered-io, public-type-surface-matches-runtime, streaming-write-sheet-protection-before-autofilter, write-buffer-return-type-contract.