Adds l4d2web/services/overlay_builders.py with a BUILDERS dict mapping
Overlay.type to a builder class. ExternalBuilder is a no-op that just
ensures the overlay directory exists. WorkshopBuilder diff-applies
absolute symlinks under left4dead2/addons/ against the overlay's current
WorkshopItem associations: creates new ones, removes obsolete, leaves
unrelated files alone, and skips uncached items with a warning rather
than producing dangling symlinks.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
