Add module include support for mixin method resolution

[dak2]

Summary

• Add include detection in dispatch.rs to recognize module mixin declarations (single, multiple, qualified names)
• Track module inclusions in GlobalEnv via module_inclusions: HashMap<String, Vec<String>>
• Extend MethodRegistry::fallback_chain to search included modules between the class and Object, following Ruby's MRO (last included module has highest priority)
• Add 8 Rust integration tests and 6 Ruby integration tests covering basic include, multiple modules, qualified names, simultaneous include, MRO priority, and error detection

Why

include Greetable was previously treated as an unknown implicit self method call, producing false TypeError on methods defined in included modules. This is a fundamental Ruby pattern that must be supported for practical type checking.

Test plan

• Rust unit tests: MethodRegistry resolve with inclusions, MRO order, class priority over include, include before Object
• Rust integration tests: basic include, return type inference, multiple modules, class method priority, qualified modules, simultaneous include, unknown module safety, simultaneous include order
• Ruby integration tests: basic no error, receiverless call, multiple modules, simultaneous include, qualified module, method type error detection
• cd core && cargo test --lib — all tests pass
• bundle exec rake test — all tests pass

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