fix(native): narrow a var()-valued font-family to its first family

[YevheniiKotyrlo]

Problem

React Native's fontFamily is one family name, and an array is a value Fabric refuses. LightningCSS types most font-family declarations, and the compiler takes their first family. Two routes still hand React Native the whole stack on main:

• a stack LightningCSS cannot type, such as one with a trailing comma, compiles to an array;
• a stack arriving through a var(), which is how a Tailwind theme override reaches font-sans, resolves to an array at render.

Solution

narrowFontFamily reduces a stack to its first family. A stack is valid only when every entry names a family: browsers ignore a written stack with an empty entry, a number, a function or an unquoted CSS-wide keyword or default, and make a stack a variable fills that way invalid at computed-value time. So one such entry invalidates the whole stack, and an invalid stack sets nothing.

• The compiler applies it on the unparsed path. It reads the written tokens for the empty entries and reserved idents parseUnparsed drops, and leaves a stack holding a var() anywhere for render, since the variable's value can still invalidate it.
• applyValue applies it to the value a var() resolves to.

The typed path already takes the first family, which is always a name there, so it is unchanged.

Tests

• src/__tests__/compiler/font-family.test.ts: the typed and unparsed paths, each way a written stack is invalid, and a var() stack left for render wherever the var() sits.
• src/__tests__/utilities/font-family.test.ts: the reduction itself.
• src/__tests__/native/font-family-stack.test.ts: applyValue, including the delayed marker it must leave alone.
• src/__tests__/native/font-family.test.tsx: a resolved var() and an entry in it that names no family, a var() behind a family, its fallbacks, an empty fallback inside an entry, quoted names and an inline variable.
• src/__tests__/vendor/tailwind/typography.test.tsx: the Typography Font Family block, with each theme stack overridden so it reaches render.
• src/__tests__/utilities/style-descriptor.test.ts covers isStyleFunction and isStyleDescriptorArray, which fix: resolve react-native-css styling and tooling regressions on Expo 57 #451 fixed and nothing tested.
• On main, 46 of these 170 cases fail. Each of 15 mutations fails its own cases.
• Every expected value is what Chromium 153, Firefox 155 and WebKit 26.6 compute for the same CSS; React Native has no fallback list, so where browsers render a stack the case expects its first family.

Verification

On Ubuntu with the workflow's steps - build, lint, typecheck, yarn test, node --test node/__tests__/*.mjs, both library builds, the web export, the unstaged-changes checks and commitlint - everything passes.

Device evidence, before and after: #417 (comment)

Known limits

• The compiler stores a space-separated name and a comma-separated stack as the same array. An unquoted multi-word name in a variable, such as --f: Helvetica Neue, therefore reads as a stack and narrows to Helvetica. Quoting the name keeps it whole. Telling the two apart needs the variable's separators kept, which affects every var() value, so I am raising it separately.
• A var() with no value, and a CSS-wide keyword inside a variable's stack, never reach the reduction: the runtime drops a list entry that resolves to nothing, so var(--missing), Helvetica renders Helvetica where browsers inherit the family. fix: resolve CSS-wide keywords and the inherited colour as CSS defines them #391 makes such a declaration invalid at computed-value time.
• LightningCSS types font-family: Inter, inherit as two families and unquotes every name, so the typed path cannot tell inherit from "inherit" and takes Inter; browsers reject the unquoted keyword.

Merge order

It shares lines with #389 (compiler/declarations.ts), #391 (compiler/declarations.ts), #413 (compiler/declarations.ts, utilities/index.ts), #420 (compiler/declarations.ts); whichever lands second rebases.

It also shares lines with #491 (compiler/declarations.ts), #492 (compiler/declarations.ts), #494 (compiler/declarations.ts), #495 (compiler/declarations.ts); whichever lands second rebases.

Base

Re-written on main (a5002c5). #451 fixed isStyleFunction and isStyleDescriptorArray, which this PR also fixed, so that half is gone and its tests stay as coverage.

[YevheniiKotyrlo]

Device evidence — before / after

UNFIXED — Subject matches the REFERENCE — the whole stack reached the platform as one name, matched nothing, fell back.

FIXED — Subject matches the CONTROL — the stack narrowed to its first family, so both render the icon font.

before — 3.1.0-rc.0	after — with this PR

The reference row moves too: before sets it in the platform face, after in the theme’s Inter, because this PR also narrows the theme’s own var() font stack.

Android 16 emulator (API 36, GLES on the host GPU), 1140×2400 @ 480dpi, both frames from one run. before is the published 3.1.0-rc.0 plus what my test app needs from my other patches to run: the native and native-internal subpath exports, PR 459, cascade-layer ranking, and a shim that compiles a class-qualified :root inert. after adds the rest of my open PRs, this one included.