walkFiles maxResults concurrency ordering

Trap: using parallel walkFiles() with a caller that exits after a result limit looks faster, but concurrent workers may already have queued or begun extra traversal and yield completion-order results. Fix: callers requiring deterministic early-stop behavior use concurrency: 1; parallel mode is for broad scans where overlapping directory reads outweigh nondeterminism. Excluded directories such as node_modules are never opened, and followed symlinks still use visited-directory tracking to prevent re-entry.