The server tag now builds a parts list (string runs + async holes) instead
of an eager string. Sync templates collapse to one string and toString()
is byte-identical to before. Promises and async iterables in child
position make the result streamable: iterating it yields everything before
a hole immediately, then each hole as it resolves, in document order —
resolved values get full child semantics (templates, arrays, raw,
escaping). toString() on async content throws with streaming guidance;
async values in attribute position throw immediately.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014NgfSjHE11oFpoSnLVKLXd
