Handlebars: investigate incremental rendering

[alandefreitas]

We discussed incremental rendering (template-level, buffer-level, or both?), but helpers are black boxes that do not yield control mid-render.

Proposed solution

• Define the scope of incremental rendering (template vs. buffer).
• Experiment with coroutine-based helpers to support yielding.
• Document expectations for helper authors and identify limitations.
• Produce a proof-of-concept covering one approach.

Requirements

• Design document outlining viable incremental strategies.
• Prototype showing at least one strategy in action.
• Tests or demos illustrating incremental output.
• Guidance for users on opting in or staying with eager rendering.

Note
After the standalone Handlebars project is established, move this investigation there for continued work.