Bug: TokenVisitor missing visitPost breaks parent tracking

[leogdion]

Description

TokenVisitor.visitPre sets current = treeNode when entering a node, but there is no visitPost override to restore current to the parent node when leaving. After visiting a node's first child subtree, current still points to the deepest node visited, so subsequent children at the same level get incorrect parent IDs. This produces a malformed tree for any syntax node with more than one child.

Fix

Add a visitPost override that restores current to its parent:

override func visitPost(_ node: Syntax) {
    current = current?.parentNode  // or pop from a stack
}