VFS: proper .. (parent directory) tracking

[sysheap]

Currently the VFS layer does not track parent directories. .. is silently skipped in walk_with_depth() and resolve_path_nofollow() falls back to resolve_path() when encountering ... This means:

• cd /tmp/foo/.. resolves to /tmp/foo instead of /tmp
• stat("/tmp/..") returns the stat of /tmp instead of /
• Paths like /a/b/../c resolve to /a/b/c instead of /a/c

Proposed approach

Track the parent node during path resolution. Options:

1. Walk-time parent stack: Maintain a stack of parent nodes during walk_with_depth(). When encountering .., pop the stack. This is the simplest approach and doesn't require storing parent refs on nodes.

2. Per-node parent ref: Store a Weak<dyn VfsNode> parent reference on each TmpfsDir/TmpfsFile. Set on creation. More complex but enables getcwd() to walk up the tree.

Option 1 is likely sufficient — Linux also resolves .. during path walk, not via stored parent pointers.

Files involved

• kernel/src/fs/vfs.rs — walk_with_depth(), resolve_path_nofollow(), resolve_parent()
• kernel/src/syscalls/fs_ops.rs — resolve_stat_node() dirfd-relative .. handling

Current workaround (commit 42eb6f6)

.. is skipped in walk_with_depth and treated as . in resolve_path_nofollow / resolve_stat_node. This unblocks ls -alh but is semantically incorrect for programs that rely on .. for navigation.

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Created by Claude Code