Rich fences

The fence tags:

Tag	Block
`chart`	A chart drawn from block-local data.
`table`	A table drawn from block-local data.
`metric`	A single large number with its caption and change.
`callout`	A highlighted note with a direction.
`image`	An image from the report folder or a credited Unsplash photo.
`embed`	An iframe of an allowed https host.
`scrolly`	One image that stays in view beside narrative steps, each naming the state the figure takes.
`audio`	An audio player.
`video`	A video player.

Rich block bodies must be YAML mappings. Fence recognition follows CommonMark, including longer fences and quoted code examples; diagnostic lines include the frontmatter.

An unknown key in a rich block, at any depth, is a block.unknown-key warning with its line and the nearest known key, and it is dropped before rendering, so it reaches neither the HTML nor a payload. A known key with a value of the wrong type, enum or range, and a missing required field, is a block.schema error: the fence stays visible as code and publishing refuses the report. A required title (chart, audio, video) must not be empty; an image’s alt may be "" to mark it decorative. The same walker checks the frontmatter, where an unknown key warns and stays in the metadata with no effect.