Migrate from Dokany/DokanNet to WinFsp

[Copilot]

Replace DokanNet driver layer with WinFsp (winfsp.net). Includes a comprehensive migration plan for future zero-heap read path using IMemoryOwner<byte> and memory-mapped files.

Driver swap

• DokanNet 2.3.0.3 → winfsp.net 2.1.25156 in Directory.Packages.props
• WinFspFileSystemAdapter extends Fsp.FileSystemBase — maps Open/Read/ReadDirectoryEntry/GetFileInfo/GetVolumeInfo to IVirtualFileSystem, all write ops → STATUS_ACCESS_DENIED
• WinFspHostedService manages FileSystemHost mount lifecycle — FileSystemWatcher/ArchiveChangeConsolidator/reconciliation logic ported unchanged
• FileContext carries per-handle path + isDirectory through WinFsp's FileNode/FileDesc pattern

// WinFsp Read: IntPtr buffer from kernel, Marshal.Copy from ArrayPool (transitional)
public override Int32 Read(Object FileNode, Object FileDesc,
    IntPtr Buffer, UInt64 Offset, UInt32 Length, out UInt32 BytesTransferred)
{
    var ctx = (FileContext)FileNode;
    byte[] rentedArray = ArrayPool<byte>.Shared.Rent((int)Length);
    try {
        int read = _vfs.ReadFileAsync(ctx.Path, rentedArray, (long)Offset).GetAwaiter().GetResult();
        Marshal.Copy(rentedArray, 0, Buffer, Math.Min(read, (int)Length));
        BytesTransferred = (uint)Math.Min(read, (int)Length);
        return STATUS_SUCCESS;
    } finally { ArrayPool<byte>.Shared.Return(rentedArray); }
}

Telemetry

• DokanTelemetry → FileSystemTelemetry, meter ZipDrive.Dokan → ZipDrive.FileSystem

Migration plan (src/Docs/WINFSP_MIGRATION_PLAN.md)

Documents future phases:

• Phase 2: IBufferHandle / ReadFileDirectAsync for zero-copy read path through IVirtualFileSystem
• Phase 3: MmapStorageStrategy replacing MemoryStorageStrategy's byte[] with anonymous MemoryMappedFile + MmapMemoryManager for pointer-backed Memory<byte> — eliminates per-read GC allocation
• Phase 5: mmap over ChunkedDisk sparse files for large file zero-copy

Target read path (cache hit): mmap pointer → Unsafe.CopyBlock → WinFsp IntPtr — zero managed heap allocations.

Removed

• DokanFileSystemAdapter.cs, DokanHostedService.cs, DokanTelemetry.cs
• All Dokan references in source comments, CLAUDE.md, test files