fix: return toPrimitive function instead of throwing for React 19 compat

[deggertsen]

Problem

React 19's development mode introduces a new logComponentRender function that inspects component props by iterating their properties and calling Symbol.toPrimitive to convert them to loggable values. When Legend State observables are passed as props (which is a common pattern), this triggers the Proxy's get trap for Symbol.toPrimitive, which currently throws:

Uncaught Error: [legend-state] observable should not be used as a primitive.
You may have forgotten to use .get() or .peek() to get the value of the observable.
    at Object.get (index.mjs:1548:13)
    at addValueToProperties (react-dom-client.development.js:3938:63)
    at addObjectDiffToProperties (react-dom-client.development.js:4050:15)
    at logComponentRender (react-dom-client.development.js:4131:22)
    at commitPassiveMountOnFiber (react-dom-client.development.js:15470:13)

This crashes the entire component tree in React 19 dev mode via the error boundary.

Solution

Change the Symbol.toPrimitive handler in the Proxy from throwing to returning a proper toPrimitive function:

• Returns NaN for numeric hint (arithmetic operations still produce obviously wrong results)
• Returns '[Observable]' for string/default hint (string concatenation shows the observable wasn't unwrapped)
• Emits console.warn in development mode to help developers catch accidental primitive coercion

This preserves the developer experience (you still get a warning when you accidentally use an observable as a primitive) while being compatible with React 19's dev-mode inspection.

Why not keep the throw?

The throw was a dev-time guard to catch observable + 1 mistakes. But:

1. It breaks React 19 — React's internals legitimately access Symbol.toPrimitive for dev logging
2. A console.warn serves the same purpose — developers still see the warning
3. NaN/'[Observable]' return values make the bug obvious even without the warning (you'll see NaN or [Observable] in your UI)
4. Other Proxy-based state libraries (e.g., MobX, Valtio) handle this gracefully rather than throwing

Test Changes

Updated the existing "Adding observables should throw" test to verify the new behavior: warns + returns NaN instead of throwing.

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