Support class Foo = Bar / module Foo = Bar declarations in RBS

[rhiroe]

Summary

Implement RBS class alias and module alias declarations (class Foo = Bar, module Foo = Bar).

Builds on #443, which fixed the NoMethodError caused by these declarations being silently skipped without .compact. With that crash gone, the alias was still being dropped — every YAML.load / HashWithIndifferentAccess.foo etc. came back untyped. This PR makes those resolve.

After this PR, module YAML = Psych makes the following resolve identically to Psych:

• singleton method call: YAML.load(str)
• nested constant: YAML::SyntaxError
• mixin: class A; include YAML end

This also unblocks real-world RBS sources such as module YAML = Psych (commonly authored by users), module Sidekiq::ClientMiddleware = Sidekiq::ServerMiddleware (gem_rbs_collection sidekiq), and class HashWithIndifferentAccess = ActiveSupport::HashWithIndifferentAccess (gem_rbs_collection activesupport).

Approach

The change centered on Genv#resolve_cpath:

• ModuleEntity gains @alias_decls (decl ⇒ target_mod) and @alias_target, with add_alias_decl / remove_alias_decl mirroring the existing module decl lifecycle. The alias decl is also registered as a const on the outer module so const lookup of the alias name succeeds.
• Genv#resolve_cpath follows alias_target after resolving each cpath segment, with cycle detection to handle module A = B; module B = A.
• New AST nodes SigClassAliasNode / SigModuleAliasNode register the alias via the standard define / undefine / install lifecycle. They access the alias's own ModuleEntity via outer.inner_modules[cname] directly to avoid being short-circuited by resolve_cpath's alias following.
• The previously empty when RBS::AST::Declarations::AliasDecl branch in AST.create_rbs_decl is replaced with explicit when ClassAlias / when ModuleAlias branches.

Test plan

• scenario/rbs/class-module-alias.rb: class alias instance method, module alias singleton method / nested constant / mixin
• scenario/rbs/class-module-alias-nested.rb: alias declared inside a module (module Outer; module InnerAlias = Inner; end)
• scenario/rbs/class-module-alias-cycle.rb: cycle (module A = B; module B = A) does not infinite-loop
• bundle exec rake test — 417 tests, 772 assertions, 0 failures

Known limitations / follow-ups

• LSP "go to definition" on an aliased name currently jumps to the target module's declaration site, not the alias declaration site. A follow-up PR will add the alias's own source location for LSP.

[mame]

@rhiroe Thanks! The PR looks good to me. Is it still marked as Draft because you're planning to do more work on it? If not, I'll go ahead and merge.

[rhiroe]

@mame I've marked this PR as ready for review.

[mame]

Thanks!