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.inio.pas

@@ -65,7 +65,7 @@ begin
 
     { ---- READ ---- }
     if op[0] = '<' then begin
-        fHandle := driver.storage.vfs.OpenFile(absPath, omReadOnly, wmRewrite, @fError);
+        fHandle := driver.storage.vfs.OpenFile(absPath, omRead, @fError);
         if (fHandle = 0) or (fError <> eNone) then begin
             io.syslog.writestringln('Error: cannot open file for reading.');
             kfree(void(absPath));
@@ -112,10 +112,10 @@ begin
         contentLen := stringSize(content);
 
         { Try read-write rewrite first (existing file) }
-        fHandle := driver.storage.vfs.OpenFile(absPath, omReadWrite, wmRewrite, @fError);
+        fHandle := driver.storage.vfs.OpenFile(absPath, omReadWrite, @fError);
         if (fHandle = 0) or (fError <> eNone) then begin
             { Try creating new file }
-            fHandle := driver.storage.vfs.OpenFile(absPath, omWriteOnly, wmNew, @fError);
+            fHandle := driver.storage.vfs.OpenFile(absPath, omCreate, @fError);
         end;
 
         if (fHandle = 0) or (fError <> eNone) then begin