docs: DOM multi-surface rendering design proposal

[mahyarmlk]

Summary

Design proposal for true multi-surface DOM rendering. Currently renderDom only renders screenDef.surfaces[0] and ignores all other surfaces. The graph supports multiple surfaces, and the choice-form example defines two surfaces ("main" and "sidebar"), but the DOM renderer silently drops all but the first.

File changed

• docs/proposals/Dom-Multi-Surface-Rendering.md (new)

What the proposal covers

• Answers all 10 design questions from the task spec
• Defines the semantics of duplicated asks/actions across surfaces
• Surface-suffixed DOM id scheme to avoid duplicates
• Shared state and shared action execution across surface copies
• Per-surface aria-describedby, hints, blocked reasons, feedback output
• Enter key default action behavior across surfaces
• 5-phase implementation plan
• 11 test cases for the new behavior
• Full backward-compatibility guarantee for single-surface screens
• Compatibility notes for all existing examples

Review note

Docs-only PR. No runtime changes, no version bumps, no changeset.