Scale Mermaid diagrams by a percentage of their natural size (mermaid-scale)

[faysou]

Description

What I want to achieve

When rendering to Typst or PDF with mermaid-format: png, Quarto sizes each Mermaid
image at its natural size at 96 dpi. Mermaid's 16px text then prints at 12pt, which
is larger than body text in most documents, and wide diagrams run past the page.

Setting fig-width per diagram does not fix this consistently. A fixed width scales
each diagram by a different factor, so text ends up a different size in every
diagram. Matching text size requires fig-width = natural width x factor, and the
natural width is unknown until Mermaid renders the diagram.

What I need is one factor applied to every diagram's natural size, so all diagram
text prints at the same size (for example 75%, which gives 9pt text).

Proposal

A mermaid-scale option next to mermaid-format, set for the whole document or
for one cell:

---
format:
  typst:
    mermaid-format: png
    mermaid-scale: 75%
---

```{mermaid}
%%| mermaid-scale: 50%
flowchart LR
  A --> B
```

• The value is a percentage of the natural size and keeps the aspect ratio.
• An explicit fig-width or fig-height still wins, consistent with the point in
  Typst: Unable to fit mermaid diagram into columns #8543 that explicit sizes should be respected.
• It applies to the png and svg image outputs, where Quarto computes the size
  in resolveSize. With mermaid-format: js, mermaid.js sizes the diagram in the
  browser, so the option has no effect there.

This doesn't solve the column overflow in #8543 by itself, but it gives a uniform,
text-preserving way to shrink diagrams, which addresses the font-distortion concern
raised there.

Implementation

I have a working implementation on
faysou/quarto-cli@feature/mermaid-scale
(diff against main).
It validates the value in both the cell and front matter schemas, adds a changelog
entry, and includes a smoke test that compares a scaled diagram against the same
diagram at 100%, so it does not depend on font metrics.

Should I open a PR for it?