Migrate all events to IObservable<T> for pure reactive architecture

[Aaronontheweb]

Summary

This PR establishes a consistent reactive-only pattern throughout Termina by migrating all .NET events to IObservable<T> using System.Reactive Subjects.

This is a breaking change - consumers using += event subscriptions must migrate to .Subscribe().

Changes

Interface changes:

• IInvalidatingNode.Invalidated: event Action? → IObservable<Unit>

Layout nodes migrated:

• TextInputNode: Submitted, TextChanged, Invalidated → observables
• SpinnerNode, ConditionalNode, ReactiveLayoutNode: use Subject<Unit>
• ScrollableContainerNode: migrated with proper subscription management
• StreamingTextNode: ContentChanged now alias for Invalidated observable

Rendering classes migrated:

• TextInput: OnSubmit → Submitted, OnDirty → Dirty
• ScrollableContent: OnDirty → Dirty, now implements IDisposable

Documentation & guidelines:

• Added CLAUDE.md establishing architectural principle: no mixing System.Reactive with .NET events
• Updated docs and tutorials with observable patterns

Migration Guide

Before:

input.Submitted += HandleSubmit;
node.Invalidated += () => RequestRedraw();

After:

input.Submitted
    .Subscribe(HandleSubmit)
    .DisposeWith(Subscriptions);
    
node.Invalidated
    .Subscribe(_ => RequestRedraw())
    .DisposeWith(Subscriptions);

Test plan

• All 463 existing tests pass
• Demo project builds and runs correctly
• Documentation updated with new patterns

Preparatory work for #43