Improved RBS type alias support

[apiology]

Problem: solargraph typecheck --level strong rejects valid arguments when a parameter is typed by an RBS type alias.

# core/builtin.rbs
type path = string | _ToPath

# stdlib/fileutils/0/fileutils.rbs
type pathlist = ::path | Array[::path]
def self?.ln_sf: (pathlist src, ::path dest, ?noop: boolish, ?verbose: boolish) -> void

require 'fileutils'

class Repro
  # @param source [String]
  # @param dest [String]
  # @return [void]
  def link(source, dest)
    # Wrong argument type for FileUtils.ln_sf: src expected
    #   FileUtils::path, Array<FileUtils::path>, received String
    FileUtils.ln_sf(source, dest)
  end
end

String satisfies string, so pathlist accepts it; every stdlib and gem method with an alias-typed parameter is affected.

Solution: type_to_tag expands an RBS::Types::Alias to its underlying type, falling back to the nominal tag for recursive aliases and for generic ones like type box[T] = Array[T] | nil, whose args are not substituted into the expansion.

Alias names resolve against a core-aware copy of the environment, because RbsMap loads each library with core_root: nil and resolve_type_names rebinds a reference it cannot find rather than raising. That copy reparses RBS core, so it is built only for a library whose aliases show the rebinding: one of 41 here.

Test plan: FileUtils.ln_sf('a', 'b') reports 0 problems found against real stdlib RBS; the spec suite's self-contained RBS does not cover that path.

Cost: on a cold cache build of this bundle, not distinguishable from master — 113.3s against 113.7s over four interleaved runs on Ruby 3.2.6 / rbs 4.1.2.

Fixes #1255.

🤖 Generated with Claude Code