Summary

This PR implements Hindley-Milner type inference for Coral, enabling the compiler to correctly infer types for recursive functions.

Before this change:

• Recursive function calls would short-circuit and return dyn type
• Functions like factorial(n) and fibonacci(n) would be typed as dyn even though they clearly return integers

After this change:

• Recursive functions correctly infer their return types through unification
• factorial(5) is now typed as int instead of dyn
• All existing tests continue to pass

Changes

• src/infer.ml (new): Core HM inference module with type variables, substitution, and unification
• src/ast.ml: Added TVar of int to the type system for type variables
• src/semant.ml: Updated recursive call handling to use type variables and unification for branch reconciliation
• src/utilities.ml: Added TVar support and substitution field to inference state
• tests/hm-test-*.cl: New test cases for recursive type inference

Type Inference Results

Test plan

• All 125 existing tests pass
• New HM-specific tests verify correct type inference for recursive functions
• Gradual typing preserved: Dyn still works as the dynamic type escape hatch

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