AsyncTemplatePiece

A single interpolated segment in asyncTemplate.

• Primitives use String(value) for output, except that values matching value === null || value === undefined || value === false yield no text (early return in processTemplatePiece). Other falsy values such as 0 or '' are still stringified.
• Arrays are flattened recursively in order.
• Promises are awaited; the resolved value is processed the same way.
• Async iterables of primitives are streamed in order; each yielded value uses the same rules as a standalone primitive (String(...), with null / undefined / false omitted).
• Functions (thunks): if the piece is a function, it is invoked with no arguments (piece()), and the return value is processed recursively—so you can defer work or return any other AsyncTemplatePiece shape (see MaybeFn). At runtime a thunk may also return a Promise of a piece; that is covered by MaybePromise on each template interpolation.