fix: wait for complete streamed GenUI components

[SparkElf]

Summary

• collect partial-render candidates only when a top-level items[] component closes
• keep nested EChart option objects out of the render tree until their enclosing component is complete
• preserve complete bare-component roots and the bounded single-scan parser

Reproduction

A real stock DSH DOM-channel response streamed a valid full-option bar chart in this order:

1. tooltip/grid
2. xAxis
3. yAxis
4. series

The parser previously repaired step 2 into a complete EChart and ECharts threw getAxesOnZeroOf is not a function because the paired yAxis had not arrived. The same final option and bundled ECharts asset pass in Chromium when applied after the complete component exists.

Verification

• pnpm check
• regression coverage for a partial nested EChart option, a single partial EChart, and a bare component data item