fix(types): type CSS custom properties against what both platforms honour

[YevheniiKotyrlo]

Problem

vars() and <VariableContextProvider /> are typed twice, and neither type is the set of values both implementations honour:

• Web: Record<string, string | number>.
• Native: Record<string, StyleDescriptor>.

A consumer only ever sees the web types, because the root entry's declarations re-export ./web and nothing selects the native ones.

• Web types are too narrow. vars({ "--font-stack": ["Inter", "Helvetica"] }) and vars({ "--color": undefined }) are type errors, though native honours both. In src/__tests__/native/vars.test.tsx, jest resolves the native runtime and tsc the web types, so the same file passes under one and fails under the other.
• Native types are too wide for web. StyleDescriptor admits undefined, null and the compiler's var() tuples. On main, web's vars() throws on undefined and null, and stringifies a tuple as [object Object],var,y.
• Web's provider never serialises. It spreads raw values into its <div>'s style.

Solution

CustomPropertyValue is the set both platforms honour: a string, a number, a boolean, undefined, or an array of those, which is a comma-separated list.

• Both platforms type vars() and VariableContextProvider with it.
• Web serialises through one function, which leaves an undefined value unset and drops an undefined list member.
• The README shows the provider by its current name and states the type.

Tests

• src/__tests__/_custom-property-value.types.ts is type-level and runs under yarn typecheck:
  • the root, native and web entries share one parameter type;
  • each accepted shape is accepted;
  • null, an object, a var() tuple and the whole StyleDescriptor are rejected.
• src/__tests__/web/variables.test.tsx: web serialisation of lists, booleans and undefined, through vars() and the provider.
• src/__tests__/native/vars.test.tsx: a list and an undefined value reach the native runtime as typed. The undefined case keeps its variable (inlineVariables: false), so the class's value reaches the element only because an undefined entry plants no key.
• On main, 4 of the 5 web cases fail, and the native cases fail under tsc. Each of 4 mutations fails its own cases, and adding null to the type fails the typecheck.

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: #423 (comment)

Merge order

This carries #431, which makes an undefined entry plant no key on native — the native half of what this type promises — so #431 lands first.

It also shares lines with #412 (__tests__/native/vars.test.tsx, native-internal/variables.tsx), #443 (__tests__/native/vars.test.tsx), #469 (__tests__/native/vars.test.tsx); whichever lands second rebases.

Base

Re-written on main (a5002c5), where native vars() declares its ViewStyle return, which this keeps.

[YevheniiKotyrlo]

Device evidence — before / after

UNFIXED — The same image. The narrowing is types-only on native; what the type gets wrong is read between the platforms, not between the builds.

FIXED — The string row paints on both platforms. The number row paints a 48dp stub on native and runs full width on web — one permitted value, honoured by one plane.

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

Apart from the typeface, the two frames are the same image: the change is to the declarations.

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. One of them shows on every screen: PR 417 narrows a var()-valued font-family to one family, so the after sets its text in the theme's face where the before falls back to the platform's.