Mermaid `fig-cap` renders as `<p>` instead of `<figcaption>` when label is non-`fig-*`

I have:

• searched the issue tracker for similar issues.
• installed the latest version of Quarto CLI.
• formatted my issue following the Bug Reports guide.

Bug description

Mermaid code cells with fig-cap are rendered as a semantic figure when there is no label, but adding a non-crossref label (for example label: my-diag) changes the caption output from <figcaption> to a plain <p>.
This causes inconsistent HTML semantics and styling for what is still a figure-like output.

Initially from:

• #13933

Steps to reproduce

---
title: "Mermaid caption semantics with non-fig label"
format: html
---

No label:

```{mermaid}
%%| fig-cap: "Diagram Caption"
flowchart TD
   A([Test])
```

Non-fig label:

```{mermaid}
%%| fig-cap: "Diagram Caption"
%%| label: my-diag
flowchart TD
   A([Test])
```

1. Render the document to HTML with quarto render.
2. Inspect the two rendered Mermaid blocks in the HTML output.

Actual behavior

The first block (caption only) renders with <figure> and <figcaption>.
The second block (caption plus label: my-diag) renders as a generic <div id="my-diag"> with caption in a <p>, with no <figcaption> element.
This happens because any label disables the fallback <figure>/<figcaption> path, but only recognised crossref labels (for example fig-*) are promoted to
figure references downstream.

Expected behavior

When fig-cap is present, caption markup should probably remain semantically consistent and use figure caption output.
Adding a non-fig-* label should not silently downgrade the caption to a plain paragraph.
If non-crossref labels are intentionally unsupported for mermaid figure, Quarto should probably warn clearly and still preserve predictable caption HTML.

Your environment

• IDE: Visual Studio Code.
• OS: macOS (Darwin).

Quarto check output

Quarto 99.9.9
[✓] Checking environment information...
     Quarto cache location: /Users/mcanouil/Library/Caches/quarto
[✓] Checking versions of quarto binary dependencies...
     Pandoc version 3.8.3: OK
     Dart Sass version 1.87.0: OK
     Deno version 2.4.5: OK
     Typst version 0.14.2: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
     Version: 99.9.9
     commit: c792ec2170a142d767b9779fc01c986f17d49454
     Path: /Users/mcanouil/Projects/quarto-dev/quarto-cli/package/dist/bin
[✓] Checking tools....................OK
     TinyTeX: v2026.02
     VeraPDF: 1.28.2
     Chromium: (not installed)
     Chrome Headless Shell: (not installed)
[✓] Checking LaTeX....................OK
     Using: TinyTex
     Path: /Users/mcanouil/Library/TinyTeX/bin/universal-darwin
     Version: 2025
[✓] Checking Chrome Headless....................OK
     Using: Chrome from QUARTO_CHROMIUM
     Path: /Applications/Brave Browser.app/Contents/MacOS/Brave Browser
[✓] Checking basic markdown render....OK
[✓] Checking R installation...........OK
     Version: 4.5.2
     Path: /Library/Frameworks/R.framework/Resources
     knitr: 1.50
     rmarkdown: 2.30
[✓] Checking Knitr engine render......OK
[✓] Checking Python 3 installation....OK
     Version: 3.14.2
     Path: /opt/homebrew/opt/python@3.14/bin/python3.14
     Jupyter: (None)

Start with the Mermaid reproduction document in the issue and render it with quarto render; inspect the generated HTML for the caption-only and non-fig label cases. Done means a fig-cap caption remains represented by a semantic <figcaption> when a non-fig-* label is present, or the behavior is clearly warned about and documented.