Recursive directory walk has no cycle or depth guard

`contentsOfDirectory(at:contentTypes:recursive:)` recurses with no visited-set and no depth cap. Your own doc comment warns "this can cause a crash if there are recursive folders such as symlinks" — and recursion is now the *default* path for folder import (`allowRecursion: true` in `ContentView`).

Rare via iOS file providers, but it's a stack-overflow crash rather than a graceful failure.

Start at contentsOfDirectory(at:contentTypes:recursive:) and inspect the recursive folder-import path in ContentView, where allowRecursion is true by default. Exercise the walk with recursive folders or symlinks and verify traversal terminates safely without a stack-overflow crash.

Written by the indexing model from the issue text.