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)

toolchain/compile_link.sh

@@ -19,4 +19,4 @@ echo " "
 LIBGCC=$(gcc -m32 -print-libgcc-file-name)
 echo "libgcc: ${LIBGCC}"
 
-ld -m elf_i386 -s --gc-sections -Ttoolchain/linker.script -o bin/kernel.bin $objstring --start-group lib/liblvgl.a ${LIBGCC} --end-group
+ld -m elf_i386 -s --gc-sections --no-warn-execstack -Ttoolchain/linker.script -o bin/kernel.bin $objstring --start-group lib/liblvgl.a ${LIBGCC} --end-group