vfs: major overhaul — device nodes, stream I/O, mode simplification

- 5-phase VFS architecture overhaul (path resolution, dir cache, watch/notify,
  symlinks, async API)
- Add character/block device node layer with /dev/null and /dev/zero built-ins
- RegisterDevice() API for custom device nodes with read/write/size callbacks
- Merge TOpenMode + TWriteMode into single TOpenMode enum
  (omRead, omWrite, omCreate, omReadWrite, omStream)
- Add stream write support: WriteFile/WriteFileAsync now accept omStream handles
  with auto-advancing offset for both device and volume FDs
- Add PPWriteOffsetHook type and writeOffsetCallback field to TFilesystem
- Add PPReadOffsetHook-based streaming reads for FAT32 and ISO9660
- Update all callers (notepad, edit, inio, wasm runner, filepicker, filedispatch,
  vterminal, stdio) to new 2-param OpenFile API
- 77 unit tests (0 failures) covering path ops, dir cache, symlinks, watch,
  device nodes, and stream read/write
- Add doc/vfs.md — comprehensive public VFS API documentation

src/driver/storage/driver.storage.fdtable.pas

@@ -41,11 +41,12 @@ type
         Directory   : pchar;       { directory path within the volume }
         FileName    : pchar;       { filename within that directory }
         OpenMode    : uint8;       { TOpenMode ordinal — avoids VFS type dependency }
-        WriteMode   : uint8;       { TWriteMode ordinal }
         DataBuffer  : puint32;     { pre-loaded data for small files, nil otherwise }
         DataSize    : uint32;      { size of pre-loaded data in bytes }
         Loaded      : boolean;     { true if pre-loaded into DataBuffer }
         StreamOff   : uint32;      { current byte offset for streaming / on-demand reads }
+        DeviceOps   : pointer;     { PVFSDeviceOps — non-nil for device FDs }
+        DeviceData  : pointer;     { opaque data passed to device callbacks }
     end;
 
     PFDTable = ^TFDTable;
@@ -113,6 +114,8 @@ begin
     entry^.Loaded := false;
     entry^.DataSize := 0;
     entry^.StreamOff := 0;
+    entry^.DeviceOps := nil;
+    entry^.DeviceData := nil;
 end;
 
 procedure fd_table_free(table : PFDTable);