parseFontFamilyValue() split the family stack on every comma, so
`font-family: var(--brand-font, inherit)` became two tokens:
`var(--brand-font` and `inherit)`. The var() guard from #1655 only
skips tokens starting with `var(`, so the orphan fragment was treated
as a requested family, failed every resolution path, and aborted
fail-closed distributed renders with:
FontFetchError: [Compiler] Unresolved fonts in fail-closed mode:
inherit). Distributed renders require all fonts to be resolvable.
Split on top-level commas only, so a var() expression (including a
nested one) stays a single token. Quotes are tracked as well, both so
parentheses inside a quoted family name cannot skew the depth counter
and so a legal quoted comma no longer splits.
Closes#3066
