Add receiverless method call support

[dak2]

Ruby methods are frequently called without an explicit receiver (e.g., name instead of self.name). Without handling these, many method return types could not be resolved by the type inference engine.

Changes:

• Add ImplicitSelfCall variant to NeedsChildKind
• Dispatch receiver-less CallNode as ImplicitSelfCall, synthesizing an implicit self receiver from the current class/module scope (falls back to Object at top-level)
• Skip attr_reader/attr_writer/attr_accessor (deferred to next phase)
• Extract process_method_call_common to share logic between MethodCall and ImplicitSelfCall
• Hoist block/arguments extraction before receiver branch to eliminate duplication
• Add 8 tests covering: basic resolution, arguments, nested class, module, block scope, top-level, attr_* skip, and super independence