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.
• 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 Windows with Node 26:

• yarn lint clean
• yarn typecheck clean
• yarn test --coverage: every failure also fails on main on this machine (the four babel cases)
• yarn build clean
• yarn example expo export --platform web exported

Merge order

#431 changes native vars() on the same line, so whichever lands second takes both changes.

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 — stock 3.0.7	after — with this PR

Both frames come from the same device in the same run (Android 36 emulator, 1140×2400 @ 480dpi). before is stock 3.0.7 rather than "this build minus this PR", so one unrelated difference is visible and worth naming rather than leaving you to spot it: the compiler's inlineRem defaults to 14 and our build sets it to 16, so every rem-derived length in the before frame renders at 87.5% of the after one — smaller type, tighter spacing, a shorter probe box. That is a different fix, not this one. Read the pair as the subject moving against the control the scene renders beside it, which this PR does not change.

Each frame carries a build-probe width=<dp> line — a rem-derived box that resolves differently on a patched build. The capture harness reads it off the device and refuses to save a frame whose probe disagrees with the variant it claims, so a before image cannot silently be a second after.