fix: FAT32 >1024 byte read/write truncation, VFS write notifications & dir watch

- FAT32 writeFile: replace hardcoded spc=4 with bootRecord^.spc for
  cluster allocation and per-sector write loop
- FAT32 writeFile: update directory entry byteSize on disk after write
- FAT32 readFile: use dir^.byteSize instead of cluster-based estimate,
  fix off-by-one in read loop, remove spurious +sectorSize allocation
- VFS: fire VFS_OnMutation on sync/async writes (cache invalidate + watch)
- VFS: invalidate all cache entries for volume on mutation (relPath mismatch)
- VFS: add vfsParentDir helper, store VFSDir in file descriptors
- FDTable: add VFSDir field, free on close
- File browser: enable directory watch & poll timer, two-step new file
  creation (open + write), remove io.syslog import
- Linker: add --no-warn-execstack to suppress ld exit code 1 on warning
- Add core.strings.helpers unit (getFileExtension, fmtFileSize, sort)

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

@@ -40,6 +40,7 @@ type
         Volume      : PStorage_Volume;
         Directory   : pchar;       { directory path within the volume }
         FileName    : pchar;       { filename within that directory }
+        VFSDir      : pchar;       { full VFS parent directory path, e.g. '/sys' }
         OpenMode    : uint8;       { TOpenMode ordinal — avoids VFS type dependency }
         DataBuffer  : puint32;     { pre-loaded data for small files, nil otherwise }
         DataSize    : uint32;      { size of pre-loaded data in bytes }
@@ -109,6 +110,10 @@ begin
         kfree(void(entry^.FileName));
         entry^.FileName := nil;
     end;
+    if entry^.VFSDir <> nil then begin
+        kfree(void(entry^.VFSDir));
+        entry^.VFSDir := nil;
+    end;
 
     entry^.InUse := false;
     entry^.Loaded := false;