Implement fallback console (polling) for unsupported platforms

[Aaronontheweb]

Parent: #77

Summary

Implement FallbackConsole that wraps the existing polling-based logic for platforms where native P/Invoke isn't available.

Files to Create

• src/Termina/Platform/FallbackConsole.cs

Implementation

Essentially wrap the current ConsoleInputSource logic:

public sealed class FallbackConsole : IPlatformConsole
{
    public async Task<ConsoleInputEvent?> ReadInputAsync(CancellationToken ct)
    {
        while (!ct.IsCancellationRequested)
        {
            if (Console.KeyAvailable)
            {
                var key = Console.ReadKey(intercept: true);
                return new KeyEvent(key);
            }
            
            // Check for resize
            var (w, h) = GetSize();
            if (w != _lastWidth || h != _lastHeight)
            {
                _lastWidth = w;
                _lastHeight = h;
                return new ResizeEvent(w, h);
            }
            
            await Task.Delay(1, ct);  // Reduced from 10ms
        }
        return null;
    }
}

Key Differences from Current

1. Reduced polling delay (1ms instead of 10ms)
2. Implements IPlatformConsole interface
3. Returns ConsoleInputEvent records instead of writing to channel directly

Acceptance Criteria

• Implements IPlatformConsole interface
• Works on any .NET-supported platform
• Reduced polling delay for lower latency
• Resize detection via polling
• Graceful cancellation

[Aaronontheweb]

Resolved by PR #90 - Fallback console (polling) for unsupported platforms implemented.