fix: diskfs kfree-on-static-root, mount syscall, user addr space 8MiB->1GiB

fix: diskfs kfree-on-static-root, mount syscall, user addr space 8MiB->1GiB

Bug 1: ls /disk heap corruption — diskfs_close_impl called kfree on
static g_root BSS variable. Added guard: skip kfree when node == g_root.

Bug 2: mount command only displayed mounts. Added SYSCALL_MOUNT (126)
with support for tmpfs and disk-based filesystems (diskfs/fat/ext2/persistfs).
Updated userspace mount to call the syscall with device, mountpoint, and
-t fstype args.

Bug 3: doom 'Unable to allocate 5 MiB' — user address space was capped
at 8 MiB (USER_STACK_BASE=0x00800000). Raised to 1 GiB (0x40000000) in
elf.c, usermode.c, and syscall_brk_impl.