Create TODO declarations for unresolved parent scopes

[Morriar]

Consider this:

class Foo::Bar
  def bar; end
end

Without this PR, if Foo can't be resolved, Bar never gets created and bar() ends up under Object.

This PR introduces TodoDeclaration, a placeholder namespace used when a parent scope can't be resolved. Rather than running the resolution queue twice (once normally, then again in a "create Todos" mode), we create Todos eagerly in a single pass by leveraging the existing constant → namespace promotion mechanism from #505.

The logic for the example above now looks like this:

1. Try to resolve Bar -> parent scope Foo is Unresolved(None) (genuinely unknown)
2. Create Foo as a Todo declaration under Object
3. Resolve Bar under Foo, enqueue its members
4. Create bar() under Foo::Bar

If a real class Foo or module Foo definition appears later, Foo is promoted from Todo to Class/Module in place — same mechanism used to promote constants to namespaces.

Unresolved outcomes with a pending linearization (Unresolved(Some(id))) are propagated as-is rather than triggering Todo creation, so we don't create premature Todos for names that might resolve once ancestor chains are fully computed.