fix: treat opaque arrays like opaque objects

[BitPhinix]

ObservableHint.opaque has no effect on arrays. updateNodes and the proxy get trap only check the hint behind isObject(), which excludes arrays. An opaque-hinted array is still walked and gets child nodes for every element.

The types already treat an opaque array as a primitive. In observable({ arr: ObservableHint.opaque([...]) }), arr[0] and arr.push don't type-check, so the runtime was the only part that disagreed.

Change

Check the opaque hint for arrays too, in the same (isObject(x) || isArray(x)) form that extractFunctionsAndComputeds already uses. Setting an opaque array now notifies listeners once and creates no child nodes. Reading an index through the proxy returns the raw element, like a property of an opaque object.

Why it matters

We hit this in a React Native app. It stored a list of Relay fragment refs with ObservableHint.opaque(messages). Each ref carries the Relay query AST, so every element grew about 1,500 child nodes. With ~1,000 loaded messages that was ~400 MB of JS heap, and each set re-diffed the whole tree, taking up to ~250 ms. That was enough to crash the app on long lists.

Testing

• Added opaque array should be handled like primitives. It fails on main (the node has children) and passes with this change.
• tsc --noEmit, ESLint and Prettier pass on the changed lines.
• bun test --timeout 50 has the same 19 failures as unmodified main, mostly sync-retry tests that run past the 50 ms timeout, so nothing new.