State that a var() fallback hex conforms to the figure rule

What does this MR do and why?

I want the docs template's figure rule to name the fallback form, because it asks for token color variables and says nothing of var(--token, #hex), while the template's own architecture figure writes every color that way. Without it, a reader can take the canonical figure as breaking the rule it illustrates.

The Figure row in templates/docs/AGENTS.md now says that a token with a CSS fallback inside var(), fill="var(--accent, #fc6d26)", is a token color and conforms, because the page's token wins and the hex renders only where the token is undefined. A bare hex outside var() does not.

References

templates/docs/site/src/pages/guides/architecture.astro, the canonical figure templates/docs/AGENTS.md tells a site to lift.

Standards affected

None.

Exemption implications

None. Figures written either way already render the same; the change is to the rule's wording.

Test plan

• just ship passes on the branch merged with main: TOTAL: 21 standard(s) passed, 0 exempt, 0 failed.
• Every fill and stroke color in the architecture figure is a var() token with a fallback, and none is a bare hex.

An AI tool wrote this change and ran the checks above.