Fix ArrayPool corruption from double-disposal in BufferedSubStream

[Copilot]

BufferedSubStream.Dispose returns a rented array to ArrayPool but lacks guard against multiple disposals. Since _cache was readonly, it couldn't be nulled after return, causing the same array to be returned twice on subsequent Dispose calls—corrupting the pool.

Changes

• Added disposal guard: _isDisposed flag prevents re-execution of disposal logic
• Made _cache nullable: Changed from readonly byte[] to byte[]? to enable nulling after pool return
• Added disposal checks: RefillCache methods throw ObjectDisposedException if called post-disposal
• Fixed condition order: Check CanSeek before accessing Position to avoid exceptions on non-seekable streams

Pattern

Follows the same disposal pattern as base class SharpCompressStream:

protected override void Dispose(bool disposing)
{
    if (_isDisposed) return;
    _isDisposed = true;
    
    if (disposing && _cache is not null)
    {
        ArrayPool<byte>.Shared.Return(_cache);
        _cache = null;
    }
    base.Dispose(disposing);
}

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