Preserve native video playback attributes during HTML conversion

## Problem

HTML-to-block conversion preserves a video source, poster, dimensions, preload, and controls, but drops native playback attributes such as `autoplay`, `loop`, `muted`, and `playsinline`. An autoplaying background video therefore imports as a paused `core/video` block even though the media asset is correctly localized.

## Reproduction

Convert:

```html

```

The resulting `core/video` block retains `src` but omits the playback attributes from both block attributes and canonical save markup.

## Expected

Preserve supported native `core/video` attributes and emit save-compatible markup so frontend playback semantics match the source without custom JavaScript. Add generic parity coverage for child `` conversion.

## AI assistance

OpenAI GPT-5.6 Sol via OpenCode diagnosed the conversion boundary and drafted this issue. Chris Huber reviewed and directed the work.

Start at the HTML-to-block conversion boundary for the core/video block and reproduce the provided video input. Trace how native playback attributes and child source elements become block attributes, then verify that autoplay, loop, muted, and playsinline appear in canonical save markup with generic child-source parity coverage.

Written by the indexing model from the issue text.