feature: add a figure block for standalone images, captions, and cross-references

[chaoxu]

Summary

Add a real figure block for standalone images, captions, labels, and future figure-level metadata.

Right now the format spec only has inline image syntax:

• FORMAT.md:48 — ![alt](src)

That is fine for lightweight inline images, but it is the wrong semantic shape for figures that need captions, numbering, layout controls, and cross-references.

Motivation

Standalone images usually behave like document blocks, not inline phrasing content.
A block form would make it possible to support:

• captions
• #fig:... labels
• figure numbering
• cross-references like [@fig:...]
• future figure metadata such as width/alignment/source attribution
• multiple images in one figure later if needed

Proposal

Keep plain ![alt](src) for simple inline usage, but add a block-level figure form for richer semantics.

A natural fit for Coflat is the existing fenced-div plugin architecture. For example:

::: {.figure #fig:architecture}
![Architecture](img/arch.png)

System architecture overview.
:::

Expected behavior

• Inline ![alt](src) remains supported.
• A standalone figure block renders as a block container.
• Caption text is editable as normal document content.
• Labels participate in numbering and cross-reference resolution.
• Rich/read/source behavior should stay coherent like other block plugins.

Why separate this from plain images

Treating every image as a block would make inline image usage awkward.
The better split is:

• inline image syntax for lightweight inline media
• figure block for semantic document figures

Relevant context

Current format/spec only documents inline images and block tables/code/etc. There is no figure-level construct yet.

[chaoxu]

Architecture decision: figure should land as a preset on top of #506, not as a separate block architecture.

The intended syntax remains Pandoc-style fenced div with title text, using the same underlying titled referencable fenced-block system as theorem environments.

So this issue is now the product slice / acceptance target for the figure preset, while #506 is the canonical architecture tracker.