Supply the bound domain to provider initialization

The spec now supplies the bound `domain` to a provider's `initialize` function, and adds an opt-in `domain-scoped` provider declaration that the API enforces by binding such a provider to at most one `domain`. This lets stateful providers (e.g. an OFREP static-context provider with a persistent cache) scope their state to a single, unambiguous `domain`.

### What needs to happen

- Pass the bound `domain` (if any) to the provider's `initialize` when it's registered via the provider mutator
- Make the bound `domain` available to provider initialization. `initialize` is a required protocol method today, so changing its signature is a breaking change; introduce it accordingly
- Add a way for a provider to declare itself `domain-scoped`
- Enforce that a `domain-scoped` provider is bound to at most one `domain`, rejecting any attempt to bind an already-bound instance to another `domain`
- Associated testing and docs

### Related

- Depends on domain support landing first: #96

### Links

- Parent issue: https://github.com/open-feature/spec/issues/403
- Spec sections: [1.1 Flag evaluation](https://openfeature.dev/specification/sections/flag-evaluation), [2.4 Initialization](https://openfeature.dev/specification/sections/providers)

Start with the provider mutator and the required provider initialize protocol method, then review the domain support from #96. Trace provider registration and binding to determine where initialization receives the domain and where domain-scoped providers are rejected. Done means the API change, enforcement, associated tests, and documentation are covered.

Written by the indexing model from the issue text.