Fix singleton class lifecycle cleanup

[st0012]

Summary

Fixes singleton class lifecycle cleanup during document deletion and re-indexing. Singleton classes are now pruned only when they are no longer anchored by definitions, members, constant references, or surviving extend mixins on their attached object.

The cleanup path is split into focused steps for method references, constant references, unmapped definitions, raw definition entries, and declaration pruning. Removability is checked through the graph so singleton classes can account for external anchors, while TODO parents are not kept alive by unresolved references alone.

The invalidation worklist now keeps declaration lifecycle modes under InvalidationItem::Declaration { id, reason }, so declaration-specific invalidation reasons stay localized instead of widening the top-level work item enum.