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/app/app.notepad.pas

@@ -372,9 +372,9 @@ begin
     buf := pchar(kalloc(FILE_BUF_SIZE));
     memset(uint32(buf), 0, FILE_BUF_SIZE);
     memcpy(uint32(txt), uint32(buf), len);
-    fHandle := driver.storage.vfs.OpenFile(state^.currentPath, omReadWrite, wmRewrite, @fError);
+    fHandle := driver.storage.vfs.OpenFile(state^.currentPath, omReadWrite, @fError);
     if fHandle = 0 then
-        fHandle := driver.storage.vfs.OpenFile(state^.currentPath, omWriteOnly, wmNew, @fError);
+        fHandle := driver.storage.vfs.OpenFile(state^.currentPath, omCreate, @fError);
     if fHandle = 0 then begin
         kfree(void(buf));
         io.syslog.logln('NOTEPAD', 'saveFile: OpenFile failed');
@@ -413,7 +413,7 @@ var
     buf     : pchar;
 begin
     debug.tracer.push_trace('notepad.loadFile');
-    fHandle := driver.storage.vfs.OpenFile(path, omReadOnly, wmRewrite, @fError);
+    fHandle := driver.storage.vfs.OpenFile(path, omRead, @fError);
     if (fHandle = 0) or (fError <> eNone) then begin
         io.syslog.logln('NOTEPAD', 'loadFile: could not open file');
         showErrorMsgbox('Open Failed', 'Could not open file for reading.');