Clean up synthetic declarations during incremental resolution

[st0012]

Summary

Stacked on #736.

• Wraps resolve() in a two-level convergence loop: the inner fixpoint resolves pending units, the outer loop runs cleanup_empty_declarations and re-enters resolution if cleanup produced new work
• Extends cleanup_empty_declarations to sweep orphaned singleton classes (no definitions, no members, no constant references, no live descendants) alongside the existing TODO cleanup
• Adds detach_from_ancestors to the REMOVE path of invalidate_declaration — cleans up stale descendant pointers so the singleton sweep can accurately identify unused singletons
• Re-enqueues owner declarations for cleanup when a child is removed and the owner becomes empty (handles TODO parents losing their last child, singletons losing all members)
• Updates has_no_backing_definitions to treat empty singletons (no definitions AND no members) as removable, while keeping populated singletons alive

Example

Given class A::B::C; class << self; def run; end; end; end, the indexer creates TODO declarations for A and A::B, plus a singleton A::B::C::<C>. If the file is deleted:

1. Invalidation removes definitions from A::B::C and detaches them from A::B::C::<C>
2. A::B::C becomes empty → removed, cascading to its singleton and parent TODOs
3. The convergence loop re-runs cleanup until no declarations are removed and no new work is generated

Without this change, the TODO parents and singleton would persist as orphans after deletion.