vfs: complete Fase 5 - /proc/mounts improvements and mountpoint validation

vfs: complete Fase 5 - /proc/mounts improvements and mountpoint validation

- Refactor vfs_mounts_read to support offset-based reading like a file
- Add escape_mount_string helper to escape special characters (space, tab, newline, backslash)
- Replace fixed 2048-byte buffer with dynamic allocation based on actual content size
- Escape spaces as \040, tabs as \011, newlines as \012, backslashes as \\
- Update proc_mounts_read to use new vfs_mounts_read signature with offset parameter
- Remove auto-mkdirp from sys_mount (vfs_mkdirp calls removed)
- Add mountpoint existence check before mount (vfs_lookup + directory check)
- Return -ENOENT if mountpoint does not exist, -ENOTDIR if not a directory
- Add heap.h include for kmalloc/kfree in fs.c
- Test: smoke test 119/119 PASS