Refinement pass:
- DotAccess: unwraps Signal/Derived before field lookup
- UnaryOp: uses unification instead of manual matching
- Call: unifies each arg with param type, applies subst to return
- List: unifies all element types (not just first)
- If/else: unifies both branches, checks condition is Bool
- When/else: unifies body with else body, checks condition
- Match: unifies all arm types for consistency
- Assign: checks assigned value compatible with variable type
- ForIn: binds iteration variable from Array element type
Tests: 39 ds-types (up from 34), 164 workspace total, 0 failures
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