Add streaming text components with two-tier buffer architecture

[Aaronontheweb]

Summary

Add streaming text infrastructure for terminal UIs with two modes of operation:

• Persisted mode (StreamMode.Persisted): Full buffer retained with scroll support, auto-scroll when at bottom, viewport-based rendering. Suitable for chat messages, logs, full LLM output.

• Windowed mode (StreamMode.Windowed): Rolling buffer (last N lines), no scroll, "ticker tape" effect. Suitable for ephemeral content like thinking indicators where only recent content matters.

Components Added

Component	Purpose
StreamMode	Enum defining retention policies
IStreamingTextBuffer	Interface for buffer abstraction
PersistedStreamBuffer	Full history with scroll state management
WindowedStreamBuffer	Rolling window with discard tracking
WordWrapper	Utility for terminal width handling
StreamingText	Spectre.Console Component that wraps buffers

Key Features

• Thread-safe buffer operations with locking
• Word wrapping with viewport width awareness
• Scroll state tracking (user-scrolled detection, auto-scroll toggle)
• Cached wrapped line counts with dirty flags for performance
• Prefix support with separate styling for chat-style UIs

Usage Examples

```csharp
// Persisted mode for chat/logs (default)
var chat = StreamingText.CreatePersisted();
chat.Append("Hello ");
chat.Append("world!\n");
chat.ScrollUp(3);

// Windowed mode for thinking indicators
var thinking = StreamingText.CreateWindowed(windowSize: 2);
thinking.Append("Processing step 1...\n");
thinking.Append("Processing step 2...\n");
thinking.Append("Processing step 3...\n"); // Step 1 is now discarded
```

Demo Application

Added a streaming chat demo using Akka.NET to simulate LLM behavior:

• LlmSimulatorActor: Actor that generates thinking tokens followed by streaming text chunks
• StreamingChatPage: Demo page with floating panel layout
• StreamingChatViewModel: Handles text input, actor messages, and UI state

The demo showcases both streaming modes:

• Chat history uses persisted mode (scrollable full output)
• Thinking indicator uses windowed mode (last 3 lines only)

Test Plan

• 89 new unit tests covering:
  • Buffer append/clear operations
  • Word wrapping edge cases (long words, whitespace, exact width)
  • Scroll behavior (up/down/top/bottom)
  • Auto-scroll toggle on user interaction
  • Windowed mode discard tracking
  • Component integration with Spectre.Console
• All 163 tests pass
• Demo builds and links with Akka.Hosting 1.5.55.1