feat: add scope() for hierarchical child scopes

[jodinathan]

Adds ServiceContainer.scope(): a child container with its OWN singleton instance cache and a parent reference. Unknown services resolve from the parent (and cache there); scope-local services passed in scope(services:) are cached only in the child, so they never leak to the parent or siblings. has() and resolveByTag() also fall back to the parent. enhance() is untouched.

[jodinathan]

@devtronic can you check this PR please

[devtronic]

@devtronic can you check this PR please

Hi @jodinathan,
thanks for contributing. I'll review the code by the end of the week 🙌

[devtronic]

@jodinathan Sorry, I haven't gotten around to it yet, and I'll be out of the office for the rest of the week. I'll take a look as soon as I get back.

[devtronic]

@jodinathan Thanks for your patience. I spent quite some time reviewing your changes, and I think the underlying issue is architectural: we're trying to solve the wrong problem.

Singleton services always resolve to the same instance, regardless of where they are resolved. Because of that, enhance() is the preferred way to create a new scope with custom services.

What your changes are actually trying to achieve is support for scoped services, which is a feature that isn't implemented yet.

Scoped services have been on my wishlist for this project for a long time, but they're more complex than they first appear. We need to ensure that a service never depends on another service with a shorter lifetime.

Consumer ↓ / Dependency →	Transient	Scoped	Singleton
Transient	✅	✅	✅
Scoped	✅	✅	✅
Singleton	⚠️	❌	✅

My suggestion would be:

• Add a new ServiceLifetime.scoped.
• Enforce the lifetime rules shown above, either during boot() or in _tryResolveInternal(). Performing the validation during boot() is more efficient because it only runs once, while _tryResolveInternal() is more robust because it also validates services registered at runtime.
• Cache scoped service instances in a separate Map (one cache per scope).
• When resolving, check first the scoped instances and then the _serviceInstances.