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.edit.pas

@@ -138,7 +138,7 @@ begin
 
     { Open for writing }
     debug.tracer.push_trace('edit.SaveFile.openRW');
-    fHandle := driver.storage.vfs.OpenFile(FilePath, omReadWrite, wmRewrite, @fError);
+    fHandle := driver.storage.vfs.OpenFile(FilePath, omReadWrite, @fError);
     if fHandle <> 0 then begin
         debug.tracer.push_trace('edit.SaveFile.writeFile');
         driver.storage.vfs.WriteFile(fHandle, 0, puint8(buf), totalLen);
@@ -149,7 +149,7 @@ begin
     end else begin
         { Try write-only for new file }
         debug.tracer.push_trace('edit.SaveFile.openWO');
-        fHandle := driver.storage.vfs.OpenFile(FilePath, omWriteOnly, wmNew, @fError);
+        fHandle := driver.storage.vfs.OpenFile(FilePath, omCreate, @fError);
         if fHandle <> 0 then begin
             debug.tracer.push_trace('edit.SaveFile.writeFileNew');
             driver.storage.vfs.WriteFile(fHandle, 0, puint8(buf), totalLen);
@@ -181,7 +181,7 @@ begin
     if FilePath = nil then exit;
 
     debug.tracer.push_trace('edit.LoadFile.openFile');
-    fHandle := driver.storage.vfs.OpenFile(FilePath, omReadOnly, wmRewrite, @fError);
+    fHandle := driver.storage.vfs.OpenFile(FilePath, omRead, @fError);
     debug.tracer.push_trace('edit.LoadFile.openFile.done');
     if (fHandle = 0) or (fError <> eNone) then begin
         io.syslog.writestringln('New file.');