feat: add keyFor for global symbol support

[marc-gavanier]

This enables compatibility with React Server Components, which require global symbols for serialization between server and client boundaries.

📖 Context

When using piqure with Next.js App Router (React Server Components), injection keys created with key() cannot be passed from Server Components to Client Components.

⚠️ Problem

React Server Components can only serialize global symbols. Local symbols created with Symbol() fail with:

Error: Only global symbols received from Symbol.for(...) can be passed to Client Components. The symbol Symbol(my-key) cannot be found among global symbols.

This happens because key() uses Symbol(description), which creates a unique local symbol on each call.

✅ Solution

Add keyFor() function that uses Symbol.for() to create global symbols. These symbols are registered in a global registry and can be serialized across server/client boundaries.

// Before (fails with RSC)
const MY_KEY = key<MyService>('my-service'):

// After (works with RSC)
const MY_KEY = keyFor<MyService>('my-service');

🤔 Why not change key() directly?

Symbol.for() uses a global registry, which means any code knowing the description can access the same symbol. This reduces isolation compared to Symbol(). Keeping both functions lets users choose based on their environment:

• key() : strict isolation, for non-RSC environments
• keyFor() : global symbols, required for RSC compatibility

📸 Screenshot

[murdos]

It makes sense.
@Gnuk : please take a look if you can :)